diff --git a/CMakeLists.txt b/CMakeLists.txt
index dcd7e9160..1679c8721 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,8 +1,7 @@
 cmake_minimum_required(VERSION 3.16.3) # Ubuntu 20.04 LTS default
 
 execute_process(
-  COMMAND cat ${CMAKE_CURRENT_SOURCE_DIR}/VERSION
-  COMMAND tr -d "\n"
+  COMMAND head -c -1 ${CMAKE_CURRENT_SOURCE_DIR}/VERSION
   OUTPUT_VARIABLE CURRENT_VERSION)
 
 project(meevax
@@ -13,8 +12,7 @@ project(meevax
 
 include(GNUInstallDirs)
 
-string(JOIN " " AGGRESSIVE_OPTIMIZATION_OPTIONS
-  # "-flto" # This optimization causes a SEGV when compiling with Clang 10.
+string(JOIN " " ${PROJECT_NAME}_RELEASE_PLUS
   # "-fmerge-all-constants" # This optimization is very effective in reducing binary size, but non-standard to the C++ standard.
   # "-march=native" # This optimization causes "Illegal instruction" error (is Valgrind's bug) on CI.
   # "-mtune=native"
@@ -23,8 +21,8 @@ string(JOIN " " AGGRESSIVE_OPTIMIZATION_OPTIONS
 set(CMAKE_CXX_EXTENSIONS OFF)
 set(CMAKE_CXX_FLAGS_DEBUG "-Og -gdwarf-4") # NOTE: The `-gdwarf-4` option is set due to the following issues with Clang 14 and Valgrind versions below 3.20: https://bugzilla.mozilla.org/show_bug.cgi?id=1758782
 set(CMAKE_CXX_FLAGS_MINSIZEREL "-Os -DNDEBUG")
-set(CMAKE_CXX_FLAGS_RELEASE "-O2 -DNDEBUG ${AGGRESSIVE_OPTIMIZATION_OPTIONS}")
-set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "-O2 -gdwarf-4 -DNDEBUG")
+set(CMAKE_CXX_FLAGS_RELEASE "-O3 -flto -DNDEBUG ${${PROJECT_NAME}_RELEASE_PLUS}")
+set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "${CMAKE_CXX_FLAGS_RELEASE} -gdwarf-4")
 set(CMAKE_CXX_FLAGS "-Wall -Wextra -Wpedantic -pipe")
 set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
diff --git a/README.md b/README.md
index 6a483660c..96d355cd0 100644
--- a/README.md
+++ b/README.md
@@ -91,7 +91,7 @@ Procedures for each standard are provided by the following R7RS-style libraries:
 cmake -B build -DCMAKE_BUILD_TYPE=Release
 cd build
 make package
-sudo apt install build/meevax_0.5.119_amd64.deb
+sudo apt install build/meevax_0.5.159_amd64.deb
 ```
 
 or
@@ -123,9 +123,9 @@ sudo rm -rf /usr/local/share/meevax
 
 | Target Name | Description
 |-------------|-------------
-| `all`       | Build shared-library `libmeevax.0.5.119.so` and executable `meevax`
+| `all`       | Build shared-library `libmeevax.0.5.159.so` and executable `meevax`
 | `test`      | Test executable `meevax`
-| `package`   | Generate debian package `meevax_0.5.119_amd64.deb`
+| `package`   | Generate debian package `meevax_0.5.159_amd64.deb`
 | `install`   | Copy files into `/usr/local` directly
 
 ## Usage
diff --git a/VERSION b/VERSION
index e8b2b30cf..7948d08ba 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-0.5.119
+0.5.159
diff --git a/basis/include/meevax/basis.hpp b/basis/include/meevax/basis.hpp
index 9fe642b3a..c983fca78 100644
--- a/basis/include/meevax/basis.hpp
+++ b/basis/include/meevax/basis.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/basis/meevax.ss b/basis/meevax.ss
index 098b20678..914877bf1 100644
--- a/basis/meevax.ss
+++ b/basis/meevax.ss
@@ -98,3 +98,45 @@
                          ((cdr before/after)))
                        (current-dynamic-extents))
              (emergency-exit . xs)))))
+
+(define-library (meevax apply)
+  (import (only (meevax core) define if lambda)
+          (only (meevax list) append null? reverse)
+          (only (meevax pair) car cdr cons))
+
+  (export apply)
+
+  (begin (define (apply f x . xs)
+           (if (null? xs)
+               (f . x)
+               ((lambda (xs)
+                  ((lambda (x)
+                     (f . x))
+                   (append (reverse (cdr xs))
+                           (car xs))))
+                (reverse (cons x xs)))))))
+
+(define-library (meevax map)
+  (import (only (meevax apply) apply)
+          (only (meevax core) define if quote)
+          (only (meevax list) memq null? reverse)
+          (only (meevax pair) car cdr cons))
+
+  (export map)
+
+  (begin (define (map f . xs)
+           (define (map f x a)
+             (if (null? x)
+                 (reverse a)
+                 (map f
+                      (cdr x)
+                      (cons (f (car x)) a))))
+           (define (map* f xs a)
+             (if (memq '() xs)
+                 (reverse a)
+                 (map* f
+                       (map cdr xs '())
+                       (cons (apply f (map car xs '())) a))))
+           (if (null? (cdr xs))
+               (map f (car xs) '())
+               (map* f xs '())))))
diff --git a/basis/r4rs.ss b/basis/r4rs.ss
index 71a31bfff..20714a17b 100644
--- a/basis/r4rs.ss
+++ b/basis/r4rs.ss
@@ -35,7 +35,8 @@
 |#
 
 (define-library (scheme r4rs)
-  (import (only (meevax boolean) boolean? not)
+  (import (only (meevax apply) apply)
+          (only (meevax boolean) boolean? not)
           (only (meevax character) char? char=? char<? char>? char<=? char>=? char-ci=? char-ci<? char-ci>? char-ci<=? char-ci>=? char-alphabetic? char-numeric? char-whitespace? char-upper-case? char-lower-case? char->integer integer->char char-upcase char-downcase)
           (only (meevax comparator) eq? eqv? equal?)
           (only (meevax complex) make-rectangular make-polar real-part imag-part magnitude angle)
@@ -44,6 +45,7 @@
           (only (meevax inexact) exp log sqrt sin cos tan asin acos atan)
           (only (meevax list) null? list? list length append reverse list-tail list-ref memq memv assq assv)
           (only (meevax macro-transformer) er-macro-transformer identifier?)
+          (only (meevax map) map)
           (only (meevax number) number? complex? real? rational? integer? exact? inexact? = < > <= >= zero? positive? negative? odd? even? max min + * - / abs quotient remainder modulo gcd lcm numerator denominator floor ceiling truncate round expt exact inexact number->string string->number)
           (only (meevax pair) pair? cons car cdr set-car! set-cdr! caar cadr cdar cddr caaar caadr cadar caddr cdaar cdadr cddar cdddr caaaar caaadr caadar caaddr cadaar cadadr caddar cadddr cdaaar cdaadr cdadar cdaddr cddaar cddadr cdddar cddddr)
           (only (meevax port) input-port? output-port? standard-input-port standard-output-port open-input-file open-output-file close eof-object?)
@@ -51,9 +53,10 @@
           (only (meevax string) string? make-string string string-length string-ref string-set! string=? string<? string>? string<=? string>=? string-ci=? string-ci<? string-ci>? string-ci<=? string-ci>=? string-append string->list list->string string-copy string-fill!)
           (only (meevax symbol) symbol? symbol->string string->symbol)
           (only (meevax vector) vector? make-vector vector vector-length vector-ref vector-set! vector->list list->vector vector-fill!)
-          (prefix (only (meevax environment) load) %)
           (prefix (meevax read) %)
           (prefix (meevax write) %)
+          (prefix (only (meevax environment) load) %)
+          (only (srfi 39) make-parameter parameterize)
           (only (srfi 45) delay force))
 
   (export quote lambda if set! cond case and or let let* letrec begin do delay
@@ -183,39 +186,6 @@
                        (else x)))
                (expand (cadr form) 0))))
 
-         (define (every f xs)
-           (if (pair? xs)
-               (and (f (car xs))
-                    (every f (cdr xs)))
-               #t))
-
-         (define (map f x . xs) ; Chibi-Scheme
-           (define (map f x a)
-             (if (pair? x)
-                 (map f
-                      (cdr x)
-                      (cons (f (car x)) a))
-                 (reverse a)))
-           (define (map* f xs a)
-             (if (every pair? xs)
-                 (map* f
-                       (map cdr xs '())
-                       (cons (apply f (map car xs '())) a))
-                 (reverse a)))
-           (if (null? xs)
-               (map f x '())
-               (map* f (cons x xs) '())))
-
-         (define (apply f x . xs) ; Chibi-Scheme
-           (letrec ((apply (lambda (f xs)
-                             (f . xs))))
-             (if (null? xs)
-                 (apply f x)
-                 ((lambda (xs)
-                    (apply f (append (reverse (cdr xs))
-                                     (car xs))))
-                  (reverse (cons x xs))))))
-
          (define-syntax let ; Chibi-Scheme
            (er-macro-transformer
              (lambda (form rename compare)
@@ -310,8 +280,7 @@
                        (else (+ 1 fx)))))
              (cond ((< y x)
                     (simplest-rational y x))
-                   ((not (< x y))
-                    (if (rational? x) x (error x)))
+                   ((not (< x y)) x)
                    ((positive? x)
                     (simplest-rational-internal x y))
                    ((negative? x)
@@ -348,25 +317,23 @@
                result))
            (call-with-output-port (open-output-file path) f))
 
-         (define %current-input-port (standard-input-port))
-
-         (define (current-input-port) %current-input-port)
-
-         (define %current-output-port (standard-output-port))
+         (define current-input-port
+           (make-parameter (standard-input-port)))
 
-         (define (current-output-port) %current-output-port)
+         (define current-output-port
+           (make-parameter (standard-output-port)))
 
          (define (with-input-from-file path thunk)
-           (let ((previous-input-port (current-input-port)))
-             (set! %current-input-port (open-input-file path))
-             (thunk)
-             (set! %current-input-port previous-input-port)))
+           (parameterize ((current-input-port (open-input-file path)))
+             (let ((result (thunk)))
+               (close-input-port (current-input-port))
+               result)))
 
          (define (with-output-to-file path thunk)
-           (let ((previous-output-port (current-output-port)))
-             (set! %current-output-port (open-output-file path))
-             (thunk)
-             (set! %current-output-port previous-output-port)))
+           (parameterize ((current-output-port (open-output-file path)))
+             (let ((result (thunk)))
+               (close-output-port (current-output-port))
+               result)))
 
          (define close-input-port close)
 
diff --git a/basis/r7rs.ss b/basis/r7rs.ss
index 22353c49b..7b5d562d4 100644
--- a/basis/r7rs.ss
+++ b/basis/r7rs.ss
@@ -1,15 +1,15 @@
 (define-library (scheme base)
   (import (only (meevax core) include include-case-insensitive)
           (only (meevax error) error-object? read-error? file-error?)
-          (only (meevax macro-transformer) er-macro-transformer)
           (only (meevax list) make-list list-copy)
+          (only (meevax macro-transformer) er-macro-transformer)
           (only (meevax number) exact-integer? exact-integer-square-root)
           (only (meevax port) binary-port? eof-object flush get-output-u8vector open-input-u8vector open-output-u8vector open? port? standard-error-port standard-input-port standard-output-port textual-port?)
-          (prefix (meevax read) %)
           (only (meevax string) string-copy!)
           (only (meevax vector homogeneous) u8vector? make-u8vector u8vector u8vector-length u8vector-ref u8vector-set! u8vector-copy u8vector-copy! u8vector-append u8vector->string string->u8vector)
           (only (meevax vector) vector-append vector-copy vector-copy! vector->string string->vector)
           (only (meevax version) features)
+          (prefix (meevax read) %)
           (prefix (meevax write) %)
           (scheme r5rs)
           (srfi 0)
@@ -250,32 +250,8 @@
 
          (define output-port-open? open?)
 
-         (define current-input-port
-           (make-parameter (standard-input-port)
-             (lambda (x)
-               (cond ((not (input-port? x))
-                      (error "not an input-port" x))
-                     ((not (input-port-open? x))
-                      (error "not an opened input-port" x))
-                     (else x)))))
-
-         (define current-output-port
-           (make-parameter (standard-output-port)
-             (lambda (x)
-               (cond ((not (output-port? x))
-                      (error "not an output-port" x))
-                     ((not (output-port-open? x))
-                      (error "not an opened output-port" x))
-                     (else x)))))
-
          (define current-error-port
-           (make-parameter (standard-error-port)
-             (lambda (x)
-               (cond ((not (output-port? x))
-                      (error "not an output-port" x))
-                     ((not (output-port-open? x))
-                      (error "not an opened output-port" x))
-                     (else x)))))
+           (make-parameter (standard-error-port)))
 
          (define (close-port x)
            (cond ((input-port? x) (close-input-port x))
@@ -440,21 +416,10 @@
   (import (only (meevax file) delete-file file-exists?)
           (only (meevax port) open-binary-input-file open-binary-output-file)
           (only (scheme base) close-input-port close-output-port current-input-port current-output-port define parameterize)
-          (only (scheme r5rs) call-with-input-file call-with-output-file open-input-file open-output-file))
-
+          (only (scheme r5rs) call-with-input-file call-with-output-file open-input-file open-output-file with-input-from-file with-output-to-file))
   (export call-with-input-file call-with-output-file delete-file file-exists?
           open-binary-input-file open-binary-output-file open-input-file
-          open-output-file with-input-from-file with-output-to-file)
-
-  (begin (define (with-input-from-file path thunk)
-           (parameterize ((current-input-port (open-input-file path)))
-             (thunk)
-             (close-input-port (current-input-port))))
-
-         (define (with-output-to-file path thunk)
-           (parameterize ((current-output-port (open-output-file path)))
-             (thunk)
-             (close-output-port (current-output-port))))))
+          open-output-file with-input-from-file with-output-to-file))
 
 (define-library (scheme inexact)
   (import (only (meevax inexact) finite? infinite? nan?)
diff --git a/basis/srfi-39.ss b/basis/srfi-39.ss
index 85abb1c21..ec61a2800 100644
--- a/basis/srfi-39.ss
+++ b/basis/srfi-39.ss
@@ -26,9 +26,8 @@
           (only (meevax core) define define-syntax if lambda letrec quote)
           (only (meevax list) null? list append assq)
           (only (meevax macro-transformer) er-macro-transformer)
-          (only (meevax pair) cons car cdr cadr cddr set-car! set-cdr!)
-          (only (scheme r5rs) map)
-          )
+          (only (meevax map) map)
+          (only (meevax pair) cons car cdr cadr cddr set-car! set-cdr!))
 
   (export make-parameter parameterize)
 
diff --git a/configure/basis.cpp b/configure/basis.cpp
index cc6bc4a33..2dc0cd7b1 100644
--- a/configure/basis.cpp
+++ b/configure/basis.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/configure/version.cpp b/configure/version.cpp
index 45847c87e..732ec34ab 100644
--- a/configure/version.cpp
+++ b/configure/version.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/bit/bit_cast.hpp b/include/meevax/bit/bit_cast.hpp
index 352214aa3..43108b9c2 100644
--- a/include/meevax/bit/bit_cast.hpp
+++ b/include/meevax/bit/bit_cast.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/bit/log2.hpp b/include/meevax/bit/log2.hpp
index a5acc705b..c6de7be60 100644
--- a/include/meevax/bit/log2.hpp
+++ b/include/meevax/bit/log2.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/bitset/simple_bitset.hpp b/include/meevax/bitset/simple_bitset.hpp
index b5f55e6aa..9ea4f7aab 100644
--- a/include/meevax/bitset/simple_bitset.hpp
+++ b/include/meevax/bitset/simple_bitset.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/chrono/duration.hpp b/include/meevax/chrono/duration.hpp
index 28e657457..4b3999961 100644
--- a/include/meevax/chrono/duration.hpp
+++ b/include/meevax/chrono/duration.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/functional/combinator.hpp b/include/meevax/functional/combinator.hpp
index 9eecbee1c..9aeec4f31 100644
--- a/include/meevax/functional/combinator.hpp
+++ b/include/meevax/functional/combinator.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/functional/compose.hpp b/include/meevax/functional/compose.hpp
index 347b792d5..ada1c2c94 100644
--- a/include/meevax/functional/compose.hpp
+++ b/include/meevax/functional/compose.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/functional/curry.hpp b/include/meevax/functional/curry.hpp
index 51387b612..b659549ca 100644
--- a/include/meevax/functional/curry.hpp
+++ b/include/meevax/functional/curry.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/iostream/escape_sequence.hpp b/include/meevax/iostream/escape_sequence.hpp
index e758263cd..0c09990a3 100644
--- a/include/meevax/iostream/escape_sequence.hpp
+++ b/include/meevax/iostream/escape_sequence.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/iostream/is_console.hpp b/include/meevax/iostream/is_console.hpp
index 55c148b66..31e1a4d18 100644
--- a/include/meevax/iostream/is_console.hpp
+++ b/include/meevax/iostream/is_console.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/iostream/lexical_cast.hpp b/include/meevax/iostream/lexical_cast.hpp
index 9539e631c..d59ada057 100644
--- a/include/meevax/iostream/lexical_cast.hpp
+++ b/include/meevax/iostream/lexical_cast.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/iterator/index_iterator.hpp b/include/meevax/iterator/index_iterator.hpp
index 06a90015a..ff367ad65 100644
--- a/include/meevax/iterator/index_iterator.hpp
+++ b/include/meevax/iterator/index_iterator.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/binary_input_file_port.hpp b/include/meevax/kernel/binary_input_file_port.hpp
index d745d6ccd..c24896966 100644
--- a/include/meevax/kernel/binary_input_file_port.hpp
+++ b/include/meevax/kernel/binary_input_file_port.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/binary_input_port.hpp b/include/meevax/kernel/binary_input_port.hpp
index 29838d3bc..07c1a27ef 100644
--- a/include/meevax/kernel/binary_input_port.hpp
+++ b/include/meevax/kernel/binary_input_port.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/binary_output_file_port.hpp b/include/meevax/kernel/binary_output_file_port.hpp
index c341a67fa..89359b3f5 100644
--- a/include/meevax/kernel/binary_output_file_port.hpp
+++ b/include/meevax/kernel/binary_output_file_port.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/binary_output_port.hpp b/include/meevax/kernel/binary_output_port.hpp
index 6ccd14f85..475bc4303 100644
--- a/include/meevax/kernel/binary_output_port.hpp
+++ b/include/meevax/kernel/binary_output_port.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/binary_port.hpp b/include/meevax/kernel/binary_port.hpp
index f0f23fe15..9665a205a 100644
--- a/include/meevax/kernel/binary_port.hpp
+++ b/include/meevax/kernel/binary_port.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/boolean.hpp b/include/meevax/kernel/boolean.hpp
index 8a0065fc6..74db373a7 100644
--- a/include/meevax/kernel/boolean.hpp
+++ b/include/meevax/kernel/boolean.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/boot.hpp b/include/meevax/kernel/boot.hpp
index 52930e1ad..d7eaa7781 100644
--- a/include/meevax/kernel/boot.hpp
+++ b/include/meevax/kernel/boot.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/box.hpp b/include/meevax/kernel/box.hpp
index 14800a058..92bcdc4fc 100644
--- a/include/meevax/kernel/box.hpp
+++ b/include/meevax/kernel/box.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -23,7 +23,7 @@ namespace meevax
 {
 inline namespace kernel
 {
-  struct box : public virtual pair // (value . unit)
+  struct box : public virtual pair // (value . null)
   {
     using pair::pair;
   };
diff --git a/include/meevax/kernel/character.hpp b/include/meevax/kernel/character.hpp
index b6a115ff5..59533cc1a 100644
--- a/include/meevax/kernel/character.hpp
+++ b/include/meevax/kernel/character.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/closure.hpp b/include/meevax/kernel/closure.hpp
index 7e0c2ffa7..1fc2bec9d 100644
--- a/include/meevax/kernel/closure.hpp
+++ b/include/meevax/kernel/closure.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/comparator.hpp b/include/meevax/kernel/comparator.hpp
index 495fd1d64..314696b73 100644
--- a/include/meevax/kernel/comparator.hpp
+++ b/include/meevax/kernel/comparator.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/complex.hpp b/include/meevax/kernel/complex.hpp
index df05495bb..e3417550f 100644
--- a/include/meevax/kernel/complex.hpp
+++ b/include/meevax/kernel/complex.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/configurator.hpp b/include/meevax/kernel/configurator.hpp
index 07b1a477a..4255e5d1d 100644
--- a/include/meevax/kernel/configurator.hpp
+++ b/include/meevax/kernel/configurator.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/continuation.hpp b/include/meevax/kernel/continuation.hpp
index bfedb7fb5..2a5762c10 100644
--- a/include/meevax/kernel/continuation.hpp
+++ b/include/meevax/kernel/continuation.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/dynamic_environment.hpp b/include/meevax/kernel/dynamic_environment.hpp
index 78c15301c..6752f26e4 100644
--- a/include/meevax/kernel/dynamic_environment.hpp
+++ b/include/meevax/kernel/dynamic_environment.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -26,7 +26,6 @@ namespace meevax
 {
 inline namespace kernel
 {
-  template <typename Environment>
   struct dynamic_environment
   {
     /*
@@ -69,16 +68,16 @@ inline namespace kernel
        `kernel-exception-handler-set!`, it is basically assumed to be set to
        R7RS Scheme's standard procedure `raise`.
     */
-    let static inline raise = unit;
+    let static inline raise = nullptr;
 
     template <typename... Ts>
-    auto apply(object const& f, Ts&&... xs) -> object
+    auto apply(object const& f, Ts&&... xs) -> decltype(auto)
     {
       s = list(f, list(std::forward<decltype(xs)>(xs)...));
-      e = unit;
+      e = nullptr;
       c = list(make(instruction::call),
                make(instruction::stop));
-      d = unit;
+      d = nullptr;
 
       return run();
     }
@@ -95,7 +94,7 @@ inline namespace kernel
       return run();
     }
 
-    auto reraise(object const& x) -> object
+    auto reraise(object const& x) -> decltype(auto)
     {
       return raise.is<null>() ? throw x : apply(raise, x);
     }
@@ -110,14 +109,6 @@ inline namespace kernel
       fetch:
         assert(c);
 
-        if constexpr (false)
-        {
-          std::cerr << faint("; s = ") << s << "\n"
-                    << faint("; e = ") << e << "\n"
-                    << faint("; c = ") << c << "\n"
-                    << faint("; d = ") << d << "\n" << std::endl;
-        }
-
         switch (car(c).template as<instruction>())
         {
         case instruction::load_absolute: /* ------------------------------------
@@ -214,7 +205,7 @@ inline namespace kernel
           *  where <closure> = (c' . e)
           *
           * ----------------------------------------------------------------- */
-          s = cons(make<closure>(cadr(c), e), s);
+          s = cons(make<closure, allocator<void>>(cadr(c), e), s);
           c = cddr(c);
           goto fetch;
 
@@ -225,7 +216,7 @@ inline namespace kernel
           *  where <continuation> = (s e c' . d)
           *
           * ----------------------------------------------------------------- */
-          s = cons(list(make<continuation>(s, e, cadr(c), d)), s);
+          s = cons(list(make<continuation, allocator<void>>(s, cons(e, cons(cadr(c), d)))), s);
           c = cddr(c);
           goto fetch;
 
@@ -247,15 +238,18 @@ inline namespace kernel
           *
           * ----------------------------------------------------------------- */
           d = cons(cdddr(c), d);
-          [[fallthrough]];
+          c = car(s) != f ? cadr(c) : caddr(c);
+          s = cdr(s);
+          goto fetch;
 
         case instruction::tail_select: /* --------------------------------------
           *
-          *  (<boolean> . s) e (%select c1 c2 . c) d => s e c' d
+          *  (<boolean> . s) e (%tail-select c1 c2) d => s e c' d
           *
           *  where c' = (if <boolean> c1 c2)
           *
           * ----------------------------------------------------------------- */
+          assert(cdddr(c).template is<null>());
           c = car(s) != f ? cadr(c) : caddr(c);
           s = cdr(s);
           goto fetch;
@@ -283,7 +277,7 @@ inline namespace kernel
             d = cons(cddr(s), e, cdr(c), d);
             c = car(callee);
             e = cons(cadr(s), cdr(callee));
-            s = unit;
+            s = nullptr;
             goto fetch;
           }
           else if (callee.is_also<primitive_procedure>()) /* -------------------
@@ -333,7 +327,7 @@ inline namespace kernel
             assert(tail(c, 1).template is<null>());
             c = car(callee);
             e = cons(cadr(s), cdr(callee));
-            s = unit;
+            s = nullptr;
             goto fetch;
           }
           else if (callee.is_also<primitive_procedure>()) /* -------------------
@@ -378,35 +372,36 @@ inline namespace kernel
           *  s e (%dummy . c) d => s (<null> . e) c d
           *
           * ----------------------------------------------------------------- */
-          e = cons(unit, e);
+          e = cons(nullptr, e);
           c = cdr(c);
           goto fetch;
 
-        case instruction::tail_letrec: /* --------------------------------------
+        case instruction::letrec: /* -------------------------------------------
           *
-          *  (<closure> xs . s) (<unit> . e) (%letrec . c) d => () (set-car! e' xs) c' d
+          *  (<closure> xs . s) (<null> . e) (%letrec . c) d => () (set-car! e' xs) c' (s e c . d)
           *
           *  where <closure> = (c' . e')
           *
           * ----------------------------------------------------------------- */
           cadar(s) = cadr(s);
+          d = cons(cddr(s), cdr(e), cdr(c), d);
           c = caar(s);
           e = cdar(s);
-          s = unit;
+          s = nullptr;
           goto fetch;
 
-        case instruction::letrec: /* -------------------------------------------
+        case instruction::tail_letrec: /* --------------------------------------
           *
-          *  (<closure> xs . s) (<unit> . e) (%letrec . c) d => () (set-car! e' xs) c' (s e c . d)
+          *  (<closure> xs . s) (<null> . e) (%tail-letrec) d => () (set-car! e' xs) c' d
           *
           *  where <closure> = (c' . e')
           *
           * ----------------------------------------------------------------- */
+          assert(cdr(c).template is<null>());
           cadar(s) = cadr(s);
-          d = cons(cddr(s), cdr(e), cdr(c), d);
           c = caar(s);
           e = cdar(s);
-          s = unit;
+          s = nullptr;
           goto fetch;
 
         case instruction::return_: /* ------------------------------------------
@@ -428,7 +423,8 @@ inline namespace kernel
           *  (x y . s) e (%cons . c) d => ((x . y) . s) e c d
           *
           * ----------------------------------------------------------------- */
-          s = cons(cons(car(s), cadr(s)), cddr(s));
+          car(s) = cons(car(s), cadr(s));
+          cdr(s) = cddr(s);
           c = cdr(c);
           goto fetch;
 
@@ -509,7 +505,7 @@ inline namespace kernel
 
         case instruction::install: /* ------------------------------------------
           *
-          *  (x . s) e (%intall i . c) d => (x . s) e c d
+          *  (x . s) e (%install i . c) d => (x . s) e c d
           *
           * ----------------------------------------------------------------- */
           assert(cadr(c).template is<exact_integer>());
diff --git a/include/meevax/kernel/environment.hpp b/include/meevax/kernel/environment.hpp
index 07c62cb80..ce348e912 100644
--- a/include/meevax/kernel/environment.hpp
+++ b/include/meevax/kernel/environment.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -19,7 +19,6 @@
 
 #include <meevax/kernel/configurator.hpp>
 #include <meevax/kernel/dynamic_environment.hpp>
-#include <meevax/kernel/optimizer.hpp>
 #include <meevax/kernel/syntactic_environment.hpp>
 
 namespace meevax
@@ -27,8 +26,7 @@ namespace meevax
 inline namespace kernel
 {
   struct environment : public configurator<environment>
-                     , public dynamic_environment<environment>
-                     , public optimizer
+                     , public dynamic_environment
                      , public syntactic_environment<environment>
   {
     using syntactic_environment::syntactic_environment;
@@ -44,8 +42,6 @@ inline namespace kernel
 
   extern template struct configurator<environment>;
 
-  extern template struct dynamic_environment<environment>;
-
   extern template struct syntactic_environment<environment>;
 } // namespace kernel
 } // namespace meevax
diff --git a/include/meevax/kernel/eof.hpp b/include/meevax/kernel/eof.hpp
index 1be6f5fc1..76e17f29f 100644
--- a/include/meevax/kernel/eof.hpp
+++ b/include/meevax/kernel/eof.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/error.hpp b/include/meevax/kernel/error.hpp
index 8f6326ea1..f441bc983 100644
--- a/include/meevax/kernel/error.hpp
+++ b/include/meevax/kernel/error.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/exact_integer.hpp b/include/meevax/kernel/exact_integer.hpp
index 2a70f15f1..3d2dff835 100644
--- a/include/meevax/kernel/exact_integer.hpp
+++ b/include/meevax/kernel/exact_integer.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/ghost.hpp b/include/meevax/kernel/ghost.hpp
index 79a467a9f..ead7c2b4f 100644
--- a/include/meevax/kernel/ghost.hpp
+++ b/include/meevax/kernel/ghost.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/homogeneous_vector.hpp b/include/meevax/kernel/homogeneous_vector.hpp
index bca80268f..5b0822600 100644
--- a/include/meevax/kernel/homogeneous_vector.hpp
+++ b/include/meevax/kernel/homogeneous_vector.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/identifier.hpp b/include/meevax/kernel/identifier.hpp
index 11bdf4ffc..de36684f2 100644
--- a/include/meevax/kernel/identifier.hpp
+++ b/include/meevax/kernel/identifier.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/identity.hpp b/include/meevax/kernel/identity.hpp
index 4b128e6ff..fcb32d145 100644
--- a/include/meevax/kernel/identity.hpp
+++ b/include/meevax/kernel/identity.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -25,7 +25,7 @@ inline namespace kernel
 {
   struct identity
   {
-    using index = std::uint32_t;
+    using index = std::int32_t;
   };
 
   struct absolute : public virtual pair // (<identifier> . <object>)
diff --git a/include/meevax/kernel/implementation_dependent.hpp b/include/meevax/kernel/implementation_dependent.hpp
index 81c8a5762..1d65f4ee0 100644
--- a/include/meevax/kernel/implementation_dependent.hpp
+++ b/include/meevax/kernel/implementation_dependent.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/include.hpp b/include/meevax/kernel/include.hpp
index 5f7393851..e0383d66c 100644
--- a/include/meevax/kernel/include.hpp
+++ b/include/meevax/kernel/include.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -23,7 +23,7 @@ namespace meevax
 {
 inline namespace kernel
 {
-  auto include(object const&, bool = true, object const& = unit) -> object;
+  auto include(object const&, bool = true, object const& = nullptr) -> object;
 } // namespace kernel
 } // namespace meevax
 
diff --git a/include/meevax/kernel/input_file_port.hpp b/include/meevax/kernel/input_file_port.hpp
index 50c88e8f4..d2685b042 100644
--- a/include/meevax/kernel/input_file_port.hpp
+++ b/include/meevax/kernel/input_file_port.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/input_homogeneous_vector_port.hpp b/include/meevax/kernel/input_homogeneous_vector_port.hpp
index e448a3eb9..d278932f9 100644
--- a/include/meevax/kernel/input_homogeneous_vector_port.hpp
+++ b/include/meevax/kernel/input_homogeneous_vector_port.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -79,7 +79,14 @@ inline namespace kernel
 
     auto peek() -> object override
     {
-      return deque.empty() ? eof_object : make<exact_integer>(deque.front());
+      if (deque.empty())
+      {
+        return eof_object;
+      }
+      else
+      {
+        return make<exact_integer>(deque.front());
+      }
     }
   };
 
diff --git a/include/meevax/kernel/input_port.hpp b/include/meevax/kernel/input_port.hpp
index b7640eb55..e6ffd6ea7 100644
--- a/include/meevax/kernel/input_port.hpp
+++ b/include/meevax/kernel/input_port.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/input_string_port.hpp b/include/meevax/kernel/input_string_port.hpp
index b3decfa02..2f1efb565 100644
--- a/include/meevax/kernel/input_string_port.hpp
+++ b/include/meevax/kernel/input_string_port.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/instruction.hpp b/include/meevax/kernel/instruction.hpp
index 9cc90487c..6847c5df4 100644
--- a/include/meevax/kernel/instruction.hpp
+++ b/include/meevax/kernel/instruction.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -17,14 +17,13 @@
 #ifndef INCLUDED_MEEVAX_KERNEL_INSTRUCTION_HPP
 #define INCLUDED_MEEVAX_KERNEL_INSTRUCTION_HPP
 
-#include <cstdint>
 #include <iostream>
 
 namespace meevax
 {
 inline namespace kernel
 {
-  enum class instruction : std::uint8_t
+  enum class instruction
   {
     call,              // a.k.a APP
     cons,              //
@@ -52,44 +51,6 @@ inline namespace kernel
   };
 
   auto operator <<(std::ostream &, instruction const&) -> std::ostream &;
-
-  constexpr auto size(instruction const& datum)
-  {
-    switch (datum)
-    {
-      case instruction::call:
-      case instruction::cons:
-      case instruction::drop:
-      case instruction::dummy:
-      case instruction::join:
-      case instruction::letrec:
-      case instruction::return_:
-      case instruction::stop:
-      case instruction::tail_call:
-      case instruction::tail_letrec:
-        return 1;
-
-      case instruction::current:
-      case instruction::install:
-      case instruction::load_absolute:
-      case instruction::load_closure:
-      case instruction::load_constant:
-      case instruction::load_continuation:
-      case instruction::load_relative:
-      case instruction::load_variadic:
-      case instruction::store_absolute:
-      case instruction::store_relative:
-      case instruction::store_variadic:
-        return 2;
-
-      case instruction::select:
-      case instruction::tail_select:
-        return 3;
-
-      default:
-        return 0;
-    }
-  }
 } // namespace kernel
 } // namespace meevax
 
diff --git a/include/meevax/kernel/interaction_environment.hpp b/include/meevax/kernel/interaction_environment.hpp
index 3fc3bcdb8..4fc778b59 100644
--- a/include/meevax/kernel/interaction_environment.hpp
+++ b/include/meevax/kernel/interaction_environment.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/library.hpp b/include/meevax/kernel/library.hpp
index ce1a56f8e..7d4c0c1fc 100644
--- a/include/meevax/kernel/library.hpp
+++ b/include/meevax/kernel/library.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -26,9 +26,9 @@ inline namespace kernel
 {
   struct library : public environment
   {
-    let declarations = unit;
+    let declarations = nullptr;
 
-    let export_specs = unit;
+    let export_specs = nullptr;
 
     template <typename F, REQUIRES(std::is_invocable<F, library &>)>
     explicit library(F declare)
diff --git a/include/meevax/kernel/list.hpp b/include/meevax/kernel/list.hpp
index fed4695b4..8b3138b05 100644
--- a/include/meevax/kernel/list.hpp
+++ b/include/meevax/kernel/list.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -26,10 +26,10 @@ inline namespace kernel
 {
   inline auto list = [](auto&&... xs) constexpr
   {
-    return (std::forward<decltype(xs)>(xs) | ... | unit);
+    return (std::forward<decltype(xs)>(xs) | ... | nullptr);
   };
 
-  auto make_list(std::size_t, object const& = unit) -> object;
+  auto make_list(std::size_t, object const& = nullptr) -> object;
 
   auto iota(std::size_t, object const& = e0, object const& = e1) -> object;
 
@@ -93,7 +93,7 @@ inline namespace kernel
 
   auto drop_right(object &, std::size_t) -> object;
 
-  auto length(object const&) -> std::size_t;
+  auto length(object const&) -> std::ptrdiff_t;
 
   auto append(object const&, object const&) -> object;
 
@@ -110,7 +110,14 @@ inline namespace kernel
   template <typename F>
   auto map(F f, object const& xs) -> object
   {
-    return xs.is<pair>() ? cons(f(car(xs)), map(f, cdr(xs))) : unit;
+    if (xs.is<pair>())
+    {
+      return cons(f(car(xs)), map(f, cdr(xs)));
+    }
+    else
+    {
+      return nullptr;
+    }
   }
 
   auto memq(object const&, object const&) -> object const&;
@@ -141,7 +148,7 @@ inline namespace kernel
     }
     else
     {
-      return unit;
+      return nullptr;
     }
   }
 
diff --git a/include/meevax/kernel/number.hpp b/include/meevax/kernel/number.hpp
index 5fce56d68..1dd0ba0d7 100644
--- a/include/meevax/kernel/number.hpp
+++ b/include/meevax/kernel/number.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/optimizer.hpp b/include/meevax/kernel/optimizer.hpp
index ec3877482..56a8066d4 100644
--- a/include/meevax/kernel/optimizer.hpp
+++ b/include/meevax/kernel/optimizer.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -17,106 +17,13 @@
 #ifndef INCLUDED_MEEVAX_KERNEL_OPTIMIZER_HPP
 #define INCLUDED_MEEVAX_KERNEL_OPTIMIZER_HPP
 
-#include <meevax/kernel/list.hpp>
+#include <meevax/kernel/pair.hpp>
 
 namespace meevax
 {
 inline namespace kernel
 {
-  struct optimizer
-  {
-    static inline auto fmerge_constants = true;
-
-    static auto merge_constants(object const& c) -> object
-    {
-      if (not c.is<pair>() or not car(c).is<instruction>())
-      {
-        return c;
-      }
-      else switch (auto n = size(car(c).as<instruction>()); car(c).as<instruction>())
-      {
-      case instruction::load_constant: /* --------------------------------------
-      *
-      *  (load-constant x
-      *   load-constant y
-      *   cons
-      *   ...)
-      *
-      *  => (load-constant (y . x)
-      *      ...)
-      *
-      * --------------------------------------------------------------------- */
-        if (tail(c, 2).is<pair>() and head(c, 2).is<instruction>() and head(c, 2).as<instruction>() == instruction::load_constant and
-            tail(c, 4).is<pair>() and head(c, 4).is<instruction>() and head(c, 4).as<instruction>() == instruction::cons)
-        {
-          return merge_constants(cons(head(c, 0),
-                                      cons(head(c, 3),
-                                           head(c, 1)),
-                                      merge_constants(tail(c, 5))));
-        }
-        else if (let const& c2 = merge_constants(tail(c, 2)); c2 == tail(c, 2))
-        {
-          return c;
-        }
-        else
-        {
-          return cons(head(c, 0), head(c, 1), c2);
-        }
-
-      default:
-        if (let const& cn = merge_constants(tail(c, n)); cn == tail(c, n))
-        {
-          return c;
-        }
-        else
-        {
-          return append(take(c, n), cn);
-        }
-
-      case instruction::load_closure:
-      case instruction::load_continuation:
-        if (let const& c1 = merge_constants(head(c, 1)),
-                       c2 = merge_constants(tail(c, 2));
-            c1 == head(c, 1) and
-            c2 == tail(c, 2))
-        {
-          return c;
-        }
-        else
-        {
-          return cons(head(c, 0), c1, c2);
-        }
-
-      case instruction::select:
-      case instruction::tail_select:
-        if (let const& c1 = merge_constants(head(c, 1)),
-                       c2 = merge_constants(head(c, 2)),
-                       c3 = merge_constants(tail(c, 3));
-            c1 == head(c, 1) and
-            c2 == head(c, 2) and
-            c3 == tail(c, 3))
-        {
-          return c;
-        }
-        else
-        {
-          return cons(head(c, 0), c1, c2, c3);
-        }
-      }
-    }
-
-    static auto optimize(object const& c)
-    {
-      let code = c;
-
-      if (fmerge_constants)
-      {
-        code = merge_constants(code);
-      }
-
-      return code;
-    }
-  };
+  auto optimize(object) -> object;
 } // namespace kernel
 } // namespace meevax
 
diff --git a/include/meevax/kernel/output_file_port.hpp b/include/meevax/kernel/output_file_port.hpp
index 21fe11adf..95000450e 100644
--- a/include/meevax/kernel/output_file_port.hpp
+++ b/include/meevax/kernel/output_file_port.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/output_homogeneous_vector_port.hpp b/include/meevax/kernel/output_homogeneous_vector_port.hpp
index 08883a4a2..5479c6d74 100644
--- a/include/meevax/kernel/output_homogeneous_vector_port.hpp
+++ b/include/meevax/kernel/output_homogeneous_vector_port.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/output_port.hpp b/include/meevax/kernel/output_port.hpp
index 60d6df414..81f94ac88 100644
--- a/include/meevax/kernel/output_port.hpp
+++ b/include/meevax/kernel/output_port.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/output_string_port.hpp b/include/meevax/kernel/output_string_port.hpp
index 0e7f7e607..3dc93dea8 100644
--- a/include/meevax/kernel/output_string_port.hpp
+++ b/include/meevax/kernel/output_string_port.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/pair.hpp b/include/meevax/kernel/pair.hpp
index a5b02366f..048f4ccce 100644
--- a/include/meevax/kernel/pair.hpp
+++ b/include/meevax/kernel/pair.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -17,44 +17,53 @@
 #ifndef INCLUDED_MEEVAX_KERNEL_PAIR_HPP
 #define INCLUDED_MEEVAX_KERNEL_PAIR_HPP
 
+#include <meevax/functional/compose.hpp>
 #include <meevax/kernel/character.hpp>
 #include <meevax/kernel/instruction.hpp>
-#include <meevax/memory/gc_pointer.hpp>
+#include <meevax/memory/allocator.hpp>
+#include <meevax/memory/collector.hpp>
 
 namespace meevax
 {
 inline namespace kernel
 {
+  using null = std::nullptr_t;
+
   struct pair;
 
-  using object = gc_pointer<pair,
-                            bool,
-                            std::int32_t, std::uint32_t,
-                            float,
-                            character,
-                            instruction>;
+  using default_collector = collector<pair, bool, int, float, character, instruction>;
+
+  using object = default_collector::mutator;
 
   using let = object;
 
   let extern unit;
 
-  template <typename T,
-            typename Allocator = collector::default_allocator<void>,
-            typename... Ts>
-  auto make(Ts&&... xs) -> object
+  template <typename T, typename Allocator, typename... Ts>
+  auto make(Ts&&... xs) -> decltype(auto)
+  {
+    return default_collector::make<T, Allocator>(std::forward<decltype(xs)>(xs)...);
+  }
+
+  template <typename T, typename... Ts>
+  auto make(Ts&&... xs) -> decltype(auto)
   {
-    return object::allocate<T, Allocator>(std::forward<decltype(xs)>(xs)...);
+    if constexpr (std::is_same_v<T, pair>) {
+      return default_collector::make<T, allocator<void>>(std::forward<decltype(xs)>(xs)...);
+    } else {
+      return default_collector::make<T, std::allocator<void>>(std::forward<decltype(xs)>(xs)...);
+    }
   }
 
   template <typename T,
-            typename Allocator = collector::default_allocator<void>>
-  auto make(T&& x) -> object
+            typename Allocator = std::allocator<void>>
+  auto make(T&& x) -> decltype(auto)
   {
-    return object::allocate<std::decay_t<T>, Allocator>(std::forward<decltype(x)>(x));
+    return default_collector::make<std::decay_t<T>, Allocator>(std::forward<decltype(x)>(x));
   }
 
   template <template <typename...> typename Traits,
-            typename Allocator = collector::default_allocator<void>,
+            typename Allocator = std::allocator<void>,
             typename... Ts,
             REQUIRES(std::is_constructible<typename Traits<Ts...>::type, Ts...>)>
   auto make(Ts&&... xs) -> decltype(auto)
@@ -62,7 +71,8 @@ inline namespace kernel
     return make<typename Traits<Ts...>::type, Allocator>(std::forward<decltype(xs)>(xs)...);
   }
 
-  struct pair : public std::pair<object, object>
+  struct pair : public virtual default_collector::top
+              , public std::pair<object, object>
   {
     template <auto Const>
     struct forward_iterator
@@ -136,51 +146,52 @@ inline namespace kernel
 
     using const_iterator = forward_iterator<true>;
 
-    constexpr pair() = default;
-
-    explicit pair(object const&);
-
-    explicit pair(object const&, object const&);
+    pair() = default;
 
-    template <typename... Ts, typename = std::enable_if_t<(1 < sizeof...(Ts))>>
-    explicit pair(object const& a, Ts&&... xs)
-      : pair { a, make<pair>(std::forward<decltype(xs)>(xs)...) }
+    template <typename T, typename U = std::nullptr_t, REQUIRES(std::is_constructible<std::pair<object, object>, T, U>)>
+    explicit pair(T&& x, U&& y = nullptr)
+      : std::pair<object, object> { std::forward<decltype(x)>(x), std::forward<decltype(y)>(y) }
     {}
 
-    virtual ~pair() = default;
+    ~pair() override = default;
 
-    virtual auto compare(pair const*) const -> bool;
+    auto compare(top const*) const -> bool override;
 
-    virtual auto type() const noexcept -> std::type_info const&;
+    auto type() const noexcept -> std::type_info const& override;
 
-    virtual auto write(std::ostream &) const -> std::ostream &;
+    auto write(std::ostream &) const -> std::ostream & override;
+
+    auto view() const noexcept -> memory::view override
+    {
+      return { this, sizeof(*this) };
+    }
 
-    constexpr auto begin() noexcept
+    auto begin() noexcept
     {
       return iterator(this);
     }
 
-    constexpr auto begin() const noexcept
+    auto begin() const noexcept
     {
       return const_iterator(this);
     }
 
-    constexpr auto end() noexcept
+    auto end() noexcept
     {
       return iterator(nullptr);
     }
 
-    constexpr auto end() const noexcept
+    auto end() const noexcept
     {
       return const_iterator(nullptr);
     }
 
-    constexpr auto cbegin() const -> const_iterator
+    auto cbegin() const -> const_iterator
     {
       return std::as_const(*this).begin();
     }
 
-    constexpr auto cend() const noexcept
+    auto cend() const noexcept
     {
       return std::as_const(*this).end();
     }
@@ -191,8 +202,7 @@ inline namespace kernel
   template <typename T, typename U, REQUIRES(std::is_constructible<pair, T, U>)>
   auto operator |(T&& x, U&& y) -> decltype(auto)
   {
-    return make<pair>(std::forward<decltype(x)>(x),
-                      std::forward<decltype(y)>(y));
+    return make<pair>(std::forward<decltype(x)>(x), std::forward<decltype(y)>(y));
   }
 
   inline auto cons = [](auto&&... xs) constexpr
@@ -265,6 +275,7 @@ inline namespace kernel
   inline constexpr auto cddddr = compose(cdr, cdddr);
 
   inline constexpr auto caddddr = compose(car, cddddr);
+  inline constexpr auto cdddddr = compose(cdr, cddddr);
 } // namespace kernel
 } // namespace meevax
 
diff --git a/include/meevax/kernel/port.hpp b/include/meevax/kernel/port.hpp
index 98d7960b9..fb31a9669 100644
--- a/include/meevax/kernel/port.hpp
+++ b/include/meevax/kernel/port.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/procedure.hpp b/include/meevax/kernel/procedure.hpp
index 338f06808..b912d73ff 100644
--- a/include/meevax/kernel/procedure.hpp
+++ b/include/meevax/kernel/procedure.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -29,6 +29,14 @@ inline namespace kernel
   {
     using signature = auto (*)(object &) -> object;
 
+    /*
+       Maybe I should use std::string_view, but I'm worried about the lifetime
+       of the referenced string. The referenced string is either a raw string
+       literal or a std::string held by symbol. In normal use, the lifetime of
+       the primitive_procedure object will not be longer than the lifetime of
+       those strings. However, it is possible to achieve this by manipulating
+       the symbol table directly.
+    */
     std::string const name;
 
     explicit primitive_procedure(std::string const& name)
@@ -45,10 +53,9 @@ inline namespace kernel
   {
     std::enable_if_t<std::is_invocable_v<F> or std::is_invocable_v<F, object &>, F> invocable;
 
-    template <typename T, typename U>
-    explicit generic_procedure(T && x, U && y)
-      : primitive_procedure { std::forward<decltype(x)>(x) }
-      , invocable           { std::forward<decltype(y)>(y) }
+    explicit generic_procedure(std::string const& name, F invocable)
+      : primitive_procedure { name }
+      , invocable           { invocable }
     {}
 
     auto operator ()(object & xs) const -> object override
diff --git a/include/meevax/kernel/ratio.hpp b/include/meevax/kernel/ratio.hpp
index a4d003a20..ef55b8bdc 100644
--- a/include/meevax/kernel/ratio.hpp
+++ b/include/meevax/kernel/ratio.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/standard_error_port.hpp b/include/meevax/kernel/standard_error_port.hpp
index c37435ca1..2d39cb900 100644
--- a/include/meevax/kernel/standard_error_port.hpp
+++ b/include/meevax/kernel/standard_error_port.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/standard_input_port.hpp b/include/meevax/kernel/standard_input_port.hpp
index a0d4b2b6f..8816f5d60 100644
--- a/include/meevax/kernel/standard_input_port.hpp
+++ b/include/meevax/kernel/standard_input_port.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/standard_output_port.hpp b/include/meevax/kernel/standard_output_port.hpp
index 776cc7fa6..ce2a775be 100644
--- a/include/meevax/kernel/standard_output_port.hpp
+++ b/include/meevax/kernel/standard_output_port.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/string.hpp b/include/meevax/kernel/string.hpp
index a9b73d5f5..8c42cbdfb 100644
--- a/include/meevax/kernel/string.hpp
+++ b/include/meevax/kernel/string.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/symbol.hpp b/include/meevax/kernel/symbol.hpp
index 41f94626c..37176adca 100644
--- a/include/meevax/kernel/symbol.hpp
+++ b/include/meevax/kernel/symbol.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/syntactic_environment.hpp b/include/meevax/kernel/syntactic_environment.hpp
index b428236ee..d111a5752 100644
--- a/include/meevax/kernel/syntactic_environment.hpp
+++ b/include/meevax/kernel/syntactic_environment.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -51,11 +51,11 @@ inline namespace kernel
                eqv(car(x).template as<syntactic_environment>()
                          .identify(cddr(x),
                                    caar(x),
-                                   unit),
+                                   nullptr),
                    car(y).template as<syntactic_environment>()
                          .identify(cddr(y),
                                    caar(y),
-                                   unit));
+                                   nullptr));
       }
 
       friend auto operator <<(std::ostream & os, syntactic_closure const& datum) -> std::ostream &
@@ -73,18 +73,17 @@ inline namespace kernel
 
     struct syntax
     {
-      using compiler = std::function<auto (syntactic_environment &,
-                                           object const& /* expression      */,
-                                           object const& /* bound_variables */,
-                                           object const& /* free_variables  */,
-                                           object const& /* continuation    */,
-                                           bool          /* tail            */) -> object>;
-
       std::string const name;
 
-      compiler const compile;
+      auto (*compile)(syntactic_environment &,
+                      object const& /* expression      */,
+                      object const& /* bound_variables */,
+                      object const& /* free_variables  */,
+                      object const& /* continuation    */,
+                      bool          /* tail            */) -> object;
 
-      explicit syntax(std::string const& name, compiler const& compile)
+      template <typename Compiler>
+      explicit syntax(std::string const& name, Compiler const& compile)
         : name { name }
         , compile { compile }
       {}
@@ -350,7 +349,7 @@ inline namespace kernel
              where <binding specs> = ((<variable 1> <initial 1>) ...
                                       (<variable n> <initial n>))
           */
-          let formals = unit;
+          let formals = nullptr;
 
           let body = sequence;
 
@@ -380,7 +379,15 @@ inline namespace kernel
           }
 
           return compile(cons(cons(rename("lambda"), formals, body),
-                              make_list(length(binding_specs), unit)),
+                              make_list(length(binding_specs), nullptr)),
+                         bound_variables,
+                         free_variables,
+                         continuation,
+                         true);
+        }
+        else if (cdr(sequence).template is<null>())
+        {
+          return compile(car(sequence),
                          bound_variables,
                          free_variables,
                          continuation,
@@ -391,14 +398,13 @@ inline namespace kernel
           return compile(car(sequence),
                          bound_variables,
                          free_variables,
-                         cdr(sequence).template is<null>() ? continuation
-                                                           : cons(make(instruction::drop),
-                                                                  body(compile,
-                                                                       cdr(sequence),
-                                                                       bound_variables,
-                                                                       free_variables,
-                                                                       continuation)),
-                         cdr(sequence).template is<null>());
+                         cons(make(instruction::drop),
+                              body(compile,
+                                   cdr(sequence),
+                                   bound_variables,
+                                   free_variables,
+                                   continuation)),
+                         false);
         }
       }
 
@@ -701,7 +707,7 @@ inline namespace kernel
         else if (let const head = compile(car(expression), // Head expression or definition
                                           bound_variables,
                                           free_variables,
-                                          unit);
+                                          nullptr);
                  head.is<null>()) // The syntax define-syntax creates a transformer from transformer-spec at compile time and registers it in the global environment. The syntax define-syntax is effectively a compile-time side-effect of the syntax environment and does nothing at run-time.
         {
           return sequence(compile,
@@ -773,7 +779,7 @@ inline namespace kernel
         return compile(cons(cons(rename("lambda"),
                                  map(formal, car(expression)), // <formals>
                                  cdr(expression)), // <body>
-                            unit), // dummy
+                            nullptr), // dummy
                        bound_variables,
                        free_variables,
                        continuation);
@@ -814,7 +820,7 @@ inline namespace kernel
         return compile(cons(cons(rename("lambda"),
                                  formals,
                                  cdr(expression)), // <body>
-                            unit), // dummy
+                            nullptr), // dummy
                        bound_variables,
                        free_variables,
                        continuation);
@@ -901,7 +907,7 @@ inline namespace kernel
       *
       * --------------------------------------------------------------------- */
       {
-        let identity = compile.identify(car(expression), unit, unit);
+        let identity = compile.identify(car(expression), nullptr, nullptr);
 
         cdr(identity) = make<transformer>(Environment().execute(compile(cadr(expression),
                                                                         bound_variables)),
@@ -962,8 +968,8 @@ inline namespace kernel
     using injector = std::function<object (object const&)>;
 
     auto operator ()(object const& expression,
-                     object const& bound_variables = unit, // list of <formals>
-                     object const& free_variables = unit,
+                     object const& bound_variables = nullptr, // list of <formals>
+                     object const& free_variables = nullptr,
                      object const& continuation = list(make(instruction::stop)),
                      bool tail = false) -> object
     {
@@ -976,7 +982,7 @@ inline namespace kernel
       *
       * --------------------------------------------------------------------- */
       {
-        return cons(make(instruction::load_constant), unit, continuation);
+        return cons(make(instruction::load_constant), nullptr, continuation);
       }
       else if (not expression.is<pair>()) /* -----------------------------------
       *
@@ -1069,6 +1075,8 @@ inline namespace kernel
         {
           return cdr(identity).as<syntax>().compile(*this, cdr(expression), bound_variables, free_variables, continuation, tail);
         }
+
+        assert(not cdr(identity).is_also<syntax>());
       }
 
       return syntax::call(*this, expression, bound_variables, free_variables, continuation, tail);
@@ -1084,8 +1092,8 @@ inline namespace kernel
 
     inline auto define(object const& variable, object const& value = undefined) -> void
     {
-      assert(identify(variable, unit, unit).template is<absolute>());
-      cdr(identify(variable, unit, unit)) = value;
+      assert(identify(variable, nullptr, nullptr).template is<absolute>());
+      cdr(identify(variable, nullptr, nullptr)) = value;
     }
 
     template <typename T, typename... Ts>
@@ -1152,7 +1160,7 @@ inline namespace kernel
           return car(variable).as<syntactic_environment>()
                               .identify(cddr(variable),
                                         unify(caar(variable), bound_variables),
-                                        unit);
+                                        nullptr);
         }
         else
         {
@@ -1218,13 +1226,13 @@ inline namespace kernel
              bind("quote-syntax"                   , syntax::quote_syntax                  ),
              bind("set!"                           , syntax::set                           )));
 
-      return make<syntactic_closure>(core_syntactic_environment, unit, make_symbol(variable));
+      return make<syntactic_closure>(core_syntactic_environment, cons(nullptr, make_symbol(variable)));
     }
 
     inline auto sweep(object const& form,
                       object const& bound_variables,
                       object const& free_variables,
-                      object const& binding_specs = unit) const -> pair
+                      object const& binding_specs = nullptr) const -> pair
     {
       if (form.is<pair>() and car(form).is<pair>())
       {
@@ -1327,7 +1335,7 @@ inline namespace kernel
 
       for (auto offset = std::max(length(a), length(b)) - length(xs); 0 < offset; --offset)
       {
-        xs = cons(unit, xs);
+        xs = cons(nullptr, xs);
       }
 
       return xs;
diff --git a/include/meevax/kernel/textual_input_port.hpp b/include/meevax/kernel/textual_input_port.hpp
index 069453bab..41a423d36 100644
--- a/include/meevax/kernel/textual_input_port.hpp
+++ b/include/meevax/kernel/textual_input_port.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/textual_output_port.hpp b/include/meevax/kernel/textual_output_port.hpp
index caaaecf9c..f8b54c596 100644
--- a/include/meevax/kernel/textual_output_port.hpp
+++ b/include/meevax/kernel/textual_output_port.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/textual_port.hpp b/include/meevax/kernel/textual_port.hpp
index e2b18dc09..522e392e6 100644
--- a/include/meevax/kernel/textual_port.hpp
+++ b/include/meevax/kernel/textual_port.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/transformer.hpp b/include/meevax/kernel/transformer.hpp
index 47e2fea30..580f709bc 100644
--- a/include/meevax/kernel/transformer.hpp
+++ b/include/meevax/kernel/transformer.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/vector.hpp b/include/meevax/kernel/vector.hpp
index 96afe341c..f7380fe76 100644
--- a/include/meevax/kernel/vector.hpp
+++ b/include/meevax/kernel/vector.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/kernel/version.hpp b/include/meevax/kernel/version.hpp
index eeb7adf1f..59fa108c1 100644
--- a/include/meevax/kernel/version.hpp
+++ b/include/meevax/kernel/version.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/map/simple_flat_map.hpp b/include/meevax/map/simple_flat_map.hpp
index d6ecec298..b833cf7c3 100644
--- a/include/meevax/map/simple_flat_map.hpp
+++ b/include/meevax/map/simple_flat_map.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/memory/allocator.hpp b/include/meevax/memory/allocator.hpp
new file mode 100644
index 000000000..cee480b6b
--- /dev/null
+++ b/include/meevax/memory/allocator.hpp
@@ -0,0 +1,129 @@
+/*
+   Copyright 2018-2024 Tatsuya Yamasaki.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+#ifndef INCLUDED_MEEVAX_MEMORY_ALLOCATOR_HPP
+#define INCLUDED_MEEVAX_MEMORY_ALLOCATOR_HPP
+
+#include <cstddef>
+#include <cstdint>
+#include <type_traits>
+#include <utility> // std::exchange
+
+namespace meevax
+{
+inline namespace memory
+{
+/*
+   Simple Segregated Storage Allocator
+*/
+template <typename T, typename Capacity = std::integral_constant<std::size_t, 1024>>
+class allocator
+{
+  struct alignas(T) chunk
+  {
+    chunk * tail;
+  };
+
+  struct block
+  {
+    static constexpr auto chunk_size = std::max(sizeof(T), sizeof(chunk));
+
+    std::uint8_t data[chunk_size * Capacity::value] = {};
+
+    std::size_t size = Capacity::value;
+
+    block * const tail;
+
+    explicit constexpr block(block * tail = nullptr)
+      : tail { tail }
+    {}
+
+    ~block()
+    {
+      delete tail;
+    }
+
+    auto pop()
+    {
+      return data + chunk_size * --size;
+    }
+  };
+
+  chunk * free_list = nullptr;
+
+  block * free_space;
+
+public:
+  using value_type = T;
+
+  using propagate_on_container_move_assignment = std::false_type;
+
+  using size_type = std::size_t;
+
+  using difference_type = std::ptrdiff_t;
+
+  using is_always_equal = std::false_type;
+
+  template <typename U>
+  struct rebind
+  {
+    using other = allocator<U, Capacity>;
+  };
+
+  explicit allocator()
+    : free_space { new block() }
+  {}
+
+  allocator(allocator &&) = delete;
+
+  allocator(allocator const&) = delete;
+
+  ~allocator()
+  {
+    delete free_space;
+  }
+
+  auto operator =(allocator &&) -> allocator & = delete;
+
+  auto operator =(allocator const&) -> allocator & = delete;
+
+  auto allocate(size_type = 1)
+  {
+    if (free_list)
+    {
+      return reinterpret_cast<value_type *>(std::exchange(free_list, free_list->tail));
+    }
+    else
+    {
+      if (not free_space->size)
+      {
+        free_space = new block(free_space);
+      }
+
+      return reinterpret_cast<value_type *>(free_space->pop());
+    }
+  }
+
+  auto deallocate(value_type * p, size_type = 1) -> void
+  {
+    reinterpret_cast<chunk *>(p)->tail = free_list;
+    free_list = reinterpret_cast<chunk *>(p);
+  }
+};
+} // namespace memory
+} // namespace meevax
+
+#endif // INCLUDED_MEEVAX_MEMORY_ALLOCATOR_HPP
diff --git a/include/meevax/memory/collector.hpp b/include/meevax/memory/collector.hpp
index 0c9675431..7936d31c2 100644
--- a/include/meevax/memory/collector.hpp
+++ b/include/meevax/memory/collector.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -17,16 +17,30 @@
 #ifndef INCLUDED_MEEVAX_MEMORY_COLLECTOR_HPP
 #define INCLUDED_MEEVAX_MEMORY_COLLECTOR_HPP
 
-#include <cstddef>
+#if __unix__
+#include <dlfcn.h> // dlopen, dlclose, dlerror
+#else
+#error
+#endif
 
+#include <memory> // std::allocator
+#include <unordered_map>
+
+#include <meevax/iostream/escape_sequence.hpp>
+#include <meevax/iostream/lexical_cast.hpp>
+#include <meevax/memory/integer_set.hpp>
 #include <meevax/memory/literal.hpp>
-#include <meevax/memory/marker.hpp>
-#include <meevax/memory/pointer_set.hpp>
+#include <meevax/memory/nan_boxing_pointer.hpp>
+#include <meevax/type_traits/is_equality_comparable.hpp>
+#include <meevax/type_traits/is_output_streamable.hpp>
+#include <meevax/utility/demangle.hpp>
 
 namespace meevax
 {
 inline namespace memory
 {
+  using view = std::pair<void const*, std::size_t>; // TODO Adapt to C++20's std::range concept
+
   /*
      This mark-and-sweep garbage collector is based on the implementation of
      gc_ptr written by William E. Kempf and posted to CodeProject.
@@ -34,76 +48,102 @@ inline namespace memory
      - https://www.codeproject.com/Articles/912/A-garbage-collection-framework-for-C
      - https://www.codeproject.com/Articles/938/A-garbage-collection-framework-for-C-Part-II
   */
+  template <typename Top, typename... Ts>
   class collector
   {
   public:
-    struct tag : public marker
+    struct top
     {
-      std::uintptr_t address;
+      virtual ~top() = default;
 
-      std::size_t size;
+      virtual auto compare(top const*) const -> bool = 0;
 
-      explicit tag(void const* const address, std::size_t size)
-        : address { reinterpret_cast<std::uintptr_t>(address) }
-        , size    { size }
-      {}
+      virtual auto type() const noexcept -> std::type_info const& = 0;
 
-      virtual ~tag() = default;
+      virtual auto write(std::ostream &) const -> std::ostream & = 0;
 
-      template <typename T = void>
-      auto lower_address() const noexcept
-      {
-        return reinterpret_cast<T *>(address);
-      }
-
-      template <typename T = void>
-      auto upper_address() const noexcept
-      {
-        return reinterpret_cast<T *>(address + size);
-      }
+      virtual auto view() const noexcept -> memory::view = 0;
 
       auto contains(void const* const data) const noexcept
       {
-        return lower_address() <= data and data < upper_address();
+        auto [address, size] = view();
+        return address <= data and data < static_cast<std::byte const*>(address) + size;
       }
     };
 
-    template <typename T, typename AllocatorTraits>
-    struct tagged : public tag
+    static inline auto cleared = false;
+
+    template <typename Bound, typename AllocatorTraits>
+    struct binder : public virtual std::conditional_t<std::is_same_v<Top, Bound>, top, Top>
+                  , public Bound
     {
-      using allocator_type = typename AllocatorTraits::template rebind_alloc<tagged<T, AllocatorTraits>>;
-
-      using pointer = typename std::allocator_traits<allocator_type>::pointer;
-
-      /*
-         Support for custom allocators is incomplete. Because the allocator is
-         templated and held as a static data member, this allocator may be
-         constructed after the collector and destructed before the collector.
-         (See "Static Initialization/Destruction Order Fiasco.") In that case,
-         illegal memory accesses will occur because the allocator corresponding
-         to a particular type has already been destructed at the time
-         collector::clear() is called within collector::~collector().
-
-         The problem is currently not occurring because the lifetime of the
-         storage allocated by std::allocator is independent of the lifetime of
-         the std::allocator type object. Conversely, if there is a relationship
-         between the storage allocated by the allocator object and the lifetime
-         of the allocator object, a problem will occur. For example, a memory
-         pool allocator, which allocates the pool in the allocator constructor
-         and releases the pool in the allocator destructor, will cause
-         problems.
-      */
-      static inline auto allocator = allocator_type();
+      struct allocator_type : public AllocatorTraits::template rebind_alloc<binder<Bound, AllocatorTraits>>
+      {
+        ~allocator_type()
+        {
+          /*
+             Execute clear before any static allocator is destroyed. Otherwise,
+             when the destructor of the collector executes clear, the collector
+             may touch the freed memory of the stateful allocator.
+          */
+          if (not std::exchange(cleared, true))
+          {
+            clear();
+          }
+        }
+      };
 
-      T body;
+      static inline auto allocator = allocator_type();
 
-      template <typename... Ts>
-      explicit tagged(Ts&&... xs)
-        : tag  { std::addressof(body), sizeof(T) }
-        , body { std::forward<decltype(xs)>(xs)... }
+      template <typename... Us>
+      explicit constexpr binder(Us&&... xs)
+        : std::conditional_t<std::is_base_of_v<Top, Bound> and std::is_constructible_v<Top, Us...>, Top, Bound> {
+            std::forward<decltype(xs)>(xs)...
+          }
       {}
 
-      ~tagged() override = default;
+      ~binder() override = default;
+
+      auto compare([[maybe_unused]] top const* other) const -> bool override
+      {
+        if constexpr (is_equality_comparable_v<Bound const&>)
+        {
+          if (auto const* bound = dynamic_cast<Bound const*>(other); bound)
+          {
+            return *bound == static_cast<Bound const&>(*this);
+          }
+          else
+          {
+            return std::is_same_v<Bound, std::nullptr_t>;
+          }
+        }
+        else
+        {
+          return false;
+        }
+      }
+
+      auto type() const noexcept -> std::type_info const& override
+      {
+        return typeid(Bound);
+      }
+
+      auto write(std::ostream & os) const -> std::ostream & override
+      {
+        if constexpr (is_output_streamable_v<Bound const&>)
+        {
+          return os << static_cast<Bound const&>(*this);
+        }
+        else
+        {
+          return os << magenta("#,(") << green(typeid(Bound).name()) << faint(" #;", static_cast<Bound const*>(this)) << magenta(")");
+        }
+      }
+
+      auto view() const noexcept -> memory::view override
+      {
+        return { this, sizeof(*this) };
+      }
 
       auto operator new(std::size_t) -> void *
       {
@@ -112,78 +152,231 @@ inline namespace memory
 
       auto operator delete(void * data) noexcept -> void
       {
+        using pointer = typename std::allocator_traits<allocator_type>::pointer;
         allocator.deallocate(reinterpret_cast<pointer>(data), 1);
       }
     };
 
-    class mutator
+    struct mutator : public nan_boxing_pointer<Top, Ts...>
     {
-      friend class collector;
+      using base_pointer = nan_boxing_pointer<Top, Ts...>;
 
-    protected:
-      tag * object = nullptr;
+      mutator(std::nullptr_t = nullptr) noexcept
+      {}
 
-      explicit constexpr mutator() = default;
+      mutator(mutator const& other)
+        : base_pointer { other }
+      {
+        if (*this)
+        {
+          mutators.insert(this);
+        }
+      }
 
-      explicit mutator(tag * object) noexcept
-        : object { object }
+      mutator(Top * top)
+        : base_pointer { top }
       {
-        if (object)
+        if (top)
         {
           mutators.insert(this);
         }
       }
 
-      ~mutator() noexcept
+      template <typename T, typename = std::enable_if_t<(std::is_same_v<T, Ts> or ... or std::is_same_v<T, double>)>>
+      mutator(T const& datum)
+        : base_pointer { datum }
+      {
+        assert(base_pointer::get() == nullptr);
+      }
+
+      ~mutator()
       {
-        if (object)
+        if (not cleared)
         {
           mutators.erase(this);
         }
       }
 
-      auto reset(tag * after = nullptr) noexcept -> void
+      auto operator =(mutator const& other) -> auto &
+      {
+        reset(other);
+        return *this;
+      }
+
+      auto operator =(std::nullptr_t) -> auto &
       {
-        if (auto before = std::exchange(object, after); not before and after)
+        reset();
+        return *this;
+      }
+
+      auto reset(mutator const& other) -> void
+      {
+        if (base_pointer::reset(other); other)
         {
           mutators.insert(this);
         }
-        else if (before and not after)
+        else
         {
           mutators.erase(this);
         }
       }
 
-      static auto locate(void * const data) noexcept -> tag *
+      auto reset(std::nullptr_t = nullptr) -> void
+      {
+        base_pointer::reset();
+        mutators.erase(this);
+      }
+
+      template <typename U>
+      inline auto as() const -> decltype(auto)
+      {
+        if constexpr (std::is_same_v<std::decay_t<U>, Top>)
+        {
+          return base_pointer::operator *();
+        }
+        else if constexpr (std::is_class_v<std::decay_t<U>>)
+        {
+          if (auto data = dynamic_cast<std::add_pointer_t<U>>(base_pointer::get()); data)
+          {
+            return *data;
+          }
+          else
+          {
+            throw std::runtime_error(lexical_cast<std::string>("no viable conversion from ", demangle(type()), " to ", demangle(typeid(U))));
+          }
+        }
+        else
+        {
+          return base_pointer::template as<U>();
+        }
+      }
+
+      template <typename U>
+      inline auto as() -> decltype(auto)
+      {
+        if constexpr (std::is_same_v<std::decay_t<U>, Top>)
+        {
+          return base_pointer::operator *();
+        }
+        else if constexpr (std::is_class_v<std::decay_t<U>>)
+        {
+          if (auto data = dynamic_cast<std::add_pointer_t<U>>(base_pointer::get()); data)
+          {
+            return *data;
+          }
+          else
+          {
+            throw std::runtime_error(lexical_cast<std::string>("no viable conversion from ", demangle(type()), " to ", demangle(typeid(U))));
+          }
+        }
+        else
+        {
+          return base_pointer::template as<U>();
+        }
+      }
+
+      template <typename U>
+      inline auto as_const() const -> decltype(auto)
+      {
+        return as<std::add_const_t<U>>();
+      }
+
+      inline auto compare(mutator const& rhs) const -> bool
       {
-        if (not data)
+        if (base_pointer::dereferenceable())
         {
-          return nullptr;
+          return *this ? base_pointer::get()->compare(rhs.get()) : rhs.is<std::nullptr_t>();
         }
-        else if (cache->contains(data)) // Heuristic-based optimization.
+        else
         {
-          return cache;
+          return base_pointer::compare(rhs);
         }
-        else if (auto iter = tags.lower_bound(reinterpret_cast<tag *>(data)); iter != tags.begin() and (*--iter)->contains(data))
+      }
+
+      template <typename U>
+      inline auto is() const
+      {
+        return type() == typeid(std::decay_t<U>);
+      }
+
+      template <typename U, REQUIRES(std::is_class<U>)>
+      inline auto is_also() const
+      {
+        return dynamic_cast<std::add_pointer_t<U>>(base_pointer::get()) != nullptr;
+      }
+
+      inline auto type() const -> std::type_info const&
+      {
+        if (base_pointer::dereferenceable())
+        {
+          return *this ? base_pointer::get()->type() : typeid(std::nullptr_t);
+        }
+        else
+        {
+          return base_pointer::type();
+        }
+      }
+
+      inline auto write(std::ostream & os) const -> std::ostream &
+      {
+        if (base_pointer::dereferenceable())
         {
-          return *iter;
+          return *this ? base_pointer::get()->write(os) : os << magenta("()");
         }
         else
         {
-          return nullptr;
+          return base_pointer::write(os);
         }
       }
+
+      friend auto operator <<(std::ostream & os, mutator const& datum) -> std::ostream &
+      {
+        return datum.write(os);
+      }
+
+      inline auto begin()
+      {
+        return *this ? base_pointer::get()->begin() : typename Top::iterator();
+      }
+
+      inline auto begin() const
+      {
+        return *this ? base_pointer::get()->cbegin() : typename Top::const_iterator();
+      }
+
+      inline auto cbegin() const
+      {
+        return *this ? base_pointer::get()->cbegin() : typename Top::const_iterator();
+      }
+
+      inline auto end()
+      {
+        return *this ? base_pointer::get()->end() : typename Top::iterator();
+      }
+
+      inline auto end() const
+      {
+        return *this ? base_pointer::get()->cend() : typename Top::const_iterator();
+      }
+
+      inline auto cend() const
+      {
+        return *this ? base_pointer::get()->cend() : typename Top::const_iterator();
+      }
     };
 
-    template <typename... Ts>
-    using default_allocator = std::allocator<Ts...>;
+    /*
+       https://www.kernel.org/doc/html/latest/arch/x86/x86_64/mm.html
 
-  protected:
-    static inline tag * cache = nullptr;
+       0x0000'0000'0000'0000 ~ 0x7FFF'FFFF'FFFF'FFFF
+    */
+    template <typename T>
+    using pointer_set = integer_set<T const*, 15, 16, 16>;
 
-    static inline v2::pointer_set<tag *> tags {};
+  private:
+    static inline pointer_set<top> objects {};
 
-    static inline v2::pointer_set<mutator *> mutators {};
+    static inline pointer_set<mutator> mutators {};
 
     static inline std::size_t allocation = 0;
 
@@ -191,59 +384,200 @@ inline namespace memory
 
     static inline std::unordered_map<std::string, std::unique_ptr<void, void (*)(void * const)>> dynamic_linked_libraries {};
 
+    struct mutators_view
+    {
+      std::uintptr_t address;
+
+      std::size_t size;
+
+      constexpr mutators_view(view const& v)
+        : address { reinterpret_cast<std::uintptr_t>(v.first) }
+        , size    { v.second }
+      {}
+
+      auto begin() const noexcept
+      {
+        return mutators.lower_bound(reinterpret_cast<mutator const*>(address));
+      }
+
+      auto end() const noexcept
+      {
+        return mutators.lower_bound(reinterpret_cast<mutator const*>(address + size));
+      }
+    };
+
   public:
-    collector() = default;
+    collector() = delete;
 
     collector(collector &&) = delete;
 
     collector(collector const&) = delete;
 
-    ~collector();
+    ~collector() = delete;
 
     auto operator =(collector &&) -> collector & = delete;
 
     auto operator =(collector const&) -> collector & = delete;
 
-    template <typename T, typename Allocator = default_allocator<void>, typename... Ts>
-    static auto make(Ts&&... xs)
+    template <typename T, typename Allocator = std::allocator<void>, typename... Us>
+    static auto make(Us&&... xs) -> mutator
     {
-      if (allocation += sizeof(T); threshold < allocation)
+      if constexpr (std::is_class_v<T>)
       {
-        collect();
+        if (allocation += sizeof(T); threshold < allocation)
+        {
+          collect();
+        }
+
+        if (auto data = new binder<T, std::allocator_traits<Allocator>>(std::forward<decltype(xs)>(xs)...); data)
+        {
+          objects.insert(data);
+
+          return data;
+        }
+        else
+        {
+          throw std::bad_alloc();
+        }
+      }
+      else
+      {
+        return { std::forward<decltype(xs)>(xs)... };
       }
+    }
 
-      if (auto data = new tagged<T, std::allocator_traits<Allocator>>(std::forward<decltype(xs)>(xs)...); data)
+    static auto clear() -> void
+    {
+      for (auto&& object : objects)
       {
-        tags.insert(cache = data);
+        delete object;
+        objects.erase(object);
+      }
+    }
 
-        return std::addressof(data->body);
+    static auto collect() -> void
+    {
+      allocation = 0;
+
+      return sweep(mark());
+    }
+
+    static auto count() noexcept -> std::size_t
+    {
+      return objects.size();
+    }
+
+    static auto dlclose(void * const handle) -> void
+    {
+      if (handle and ::dlclose(handle))
+      {
+        std::cerr << ::dlerror() << std::endl;
+      }
+    }
+
+    static auto dlopen(std::string const& filename) -> void *
+    {
+      ::dlerror(); // Clear
+
+      try
+      {
+        return dynamic_linked_libraries.at(filename).get();
+      }
+      catch (std::out_of_range const&)
+      {
+        if (auto handle = ::dlopen(filename.c_str(), RTLD_LAZY | RTLD_GLOBAL); handle)
+        {
+          dynamic_linked_libraries.emplace(std::piecewise_construct,
+                                           std::forward_as_tuple(filename),
+                                           std::forward_as_tuple(handle, dlclose));
+
+          return dlopen(filename);
+        }
+        else
+        {
+          throw std::runtime_error(::dlerror());
+        }
+      }
+    }
+
+    static auto dlsym(std::string const& symbol, void * const handle) -> void *
+    {
+      if (auto address = ::dlsym(handle, symbol.c_str()); address)
+      {
+        return address;
       }
       else
       {
-        throw std::bad_alloc();
+        throw std::runtime_error(::dlerror());
       }
     }
 
-    static auto clear() -> void;
+    static auto mark() noexcept -> pointer_set<top>
+    {
+      auto is_root_object = [begin = objects.begin()](mutator const* given) // TODO INEFFICIENT!
+      {
+        /*
+           If the given mutator is a non-root object, then an object containing
+           this mutator as a data member exists somewhere in memory.
 
-    static auto collect() -> void;
+           Containing the mutator as a data member means that the address of
+           the mutator is contained in the interval of the object's
+           base-address ~ base-address + object-size. The top is present to
+           keep track of the base-address and size of the object needed here.
+        */
+        auto iter = objects.lower_bound(reinterpret_cast<top const*>(given));
 
-    static auto count() noexcept -> std::size_t;
+        return iter == begin or not (*--iter)->contains(given);
+      };
 
-    static auto dlclose(void * const) -> void;
+      auto marked_objects = pointer_set<top>();
 
-    static auto dlopen(std::string const&) -> void *;
+      for (auto&& mutator : mutators)
+      {
+        assert(mutator);
+        assert(mutator->get());
 
-    static auto dlsym(std::string const&, void * const) -> void *;
+        if (not marked_objects.contains(mutator->get()) and is_root_object(mutator))
+        {
+          mark(mutator->get(), marked_objects);
+        }
+      }
 
-    static auto mark() -> void;
+      return marked_objects;
+    }
 
-    static auto mark(tag * const) -> void;
+    static auto mark(top const* const object, pointer_set<top> & marked_objects) noexcept -> void
+    {
+      assert(object);
 
-    static auto sweep() -> void;
-  };
+      assert(objects.contains(object));
+
+      if (not marked_objects.contains(object))
+      {
+        marked_objects.insert(object);
 
-  auto primary_collector() -> collector &;
+        for (auto each : mutators_view(object->view()))
+        {
+          mark(each->get(), marked_objects);
+        }
+      }
+    }
+
+    static auto sweep(pointer_set<top> && marked_objects) -> void
+    {
+      for (auto marked_object : marked_objects)
+      {
+        objects.erase(marked_object);
+      }
+
+      for (auto object : objects)
+      {
+        delete object;
+      }
+
+      objects.swap(marked_objects);
+    }
+  };
 } // namespace memory
 } // namespace meevax
 
diff --git a/include/meevax/memory/gc_pointer.hpp b/include/meevax/memory/gc_pointer.hpp
deleted file mode 100644
index c737a9398..000000000
--- a/include/meevax/memory/gc_pointer.hpp
+++ /dev/null
@@ -1,93 +0,0 @@
-/*
-   Copyright 2018-2023 Tatsuya Yamasaki.
-
-   Licensed under the Apache License, Version 2.0 (the "License");
-   you may not use this file except in compliance with the License.
-   You may obtain a copy of the License at
-
-       http://www.apache.org/licenses/LICENSE-2.0
-
-   Unless required by applicable law or agreed to in writing, software
-   distributed under the License is distributed on an "AS IS" BASIS,
-   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-   See the License for the specific language governing permissions and
-   limitations under the License.
-*/
-
-#ifndef INCLUDED_MEEVAX_MEMORY_GC_POINTER_HPP
-#define INCLUDED_MEEVAX_MEMORY_GC_POINTER_HPP
-
-#include <meevax/memory/collector.hpp>
-#include <meevax/memory/heterogeneous_pointer.hpp>
-#include <meevax/memory/nan_boxing_pointer.hpp>
-
-namespace meevax
-{
-inline namespace memory
-{
-  template <typename... Ts>
-  struct gc_pointer : public heterogeneous_pointer<nan_boxing_pointer, Ts...>
-                    , private collector::mutator
-  {
-    using pointer = heterogeneous_pointer<nan_boxing_pointer, Ts...>;
-
-    gc_pointer(gc_pointer const& gcp)
-      : pointer { gcp }
-      , collector::mutator { gcp.object }
-    {}
-
-    gc_pointer(pointer const& p)
-      : pointer { p }
-      , collector::mutator { locate(pointer::get()) }
-    {}
-
-    gc_pointer(std::nullptr_t = nullptr)
-    {}
-
-    template <typename T, REQUIRES(std::is_scalar<T>)>
-    explicit gc_pointer(T const& datum)
-      : pointer { datum }
-    {
-      assert(pointer::get() == nullptr);
-    }
-
-    auto operator =(gc_pointer const& gcp) -> auto &
-    {
-      reset(gcp);
-      return *this;
-    }
-
-    auto operator =(pointer const& p) -> auto &
-    {
-      reset(p);
-      return *this;
-    }
-
-    auto operator =(std::nullptr_t) -> auto &
-    {
-      reset();
-      return *this;
-    }
-
-    auto reset(gc_pointer const& gcp) -> void
-    {
-      pointer::reset(gcp);
-      collector::mutator::reset(gcp.object);
-    }
-
-    auto reset(pointer const& p) -> void
-    {
-      pointer::reset(p);
-      collector::mutator::reset(locate(pointer::get()));
-    }
-
-    auto reset(std::nullptr_t = nullptr) -> void
-    {
-      pointer::reset();
-      collector::mutator::reset();
-    }
-  };
-} // namespace memory
-} // namespace meevax
-
-#endif // INCLUDED_MEEVAX_MEMORY_GC_POINTER_HPP
diff --git a/include/meevax/memory/heterogeneous_pointer.hpp b/include/meevax/memory/heterogeneous_pointer.hpp
deleted file mode 100644
index 6a7443487..000000000
--- a/include/meevax/memory/heterogeneous_pointer.hpp
+++ /dev/null
@@ -1,256 +0,0 @@
-/*
-   Copyright 2018-2023 Tatsuya Yamasaki.
-
-   Licensed under the Apache License, Version 2.0 (the "License");
-   you may not use this file except in compliance with the License.
-   You may obtain a copy of the License at
-
-       http://www.apache.org/licenses/LICENSE-2.0
-
-   Unless required by applicable law or agreed to in writing, software
-   distributed under the License is distributed on an "AS IS" BASIS,
-   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-   See the License for the specific language governing permissions and
-   limitations under the License.
-*/
-
-#ifndef INCLUDED_MEEVAX_MEMORY_HETEROGENEOUS_POINTER_HPP
-#define INCLUDED_MEEVAX_MEMORY_HETEROGENEOUS_POINTER_HPP
-
-#include <meevax/functional/combinator.hpp>
-#include <meevax/functional/compose.hpp>
-#include <meevax/iostream/escape_sequence.hpp>
-#include <meevax/iostream/lexical_cast.hpp>
-#include <meevax/type_traits/is_equality_comparable.hpp>
-#include <meevax/type_traits/is_output_streamable.hpp>
-#include <meevax/type_traits/requires.hpp>
-#include <meevax/utility/combination.hpp>
-#include <meevax/utility/debug.hpp>
-#include <meevax/utility/demangle.hpp>
-
-namespace meevax
-{
-inline namespace memory
-{
-  using null = std::nullptr_t;
-
-  template <template <typename...> typename BasePointer, typename Top, typename... Ts>
-  class heterogeneous_pointer : public BasePointer<Top, Ts...>
-  {
-    template <typename Bound>
-    struct binder : public virtual Top
-                  , public Bound
-    {
-      template <typename... Us>
-      explicit constexpr binder(Us&&... xs)
-        : std::conditional_t<std::is_base_of_v<Top, Bound> and std::is_constructible_v<Top, Us...>, Top, Bound> {
-            std::forward<decltype(xs)>(xs)...
-          }
-      {}
-
-      ~binder() override = default;
-
-      auto compare([[maybe_unused]] Top const* top) const -> bool override
-      {
-        if constexpr (is_equality_comparable_v<Bound const&>)
-        {
-          if (auto const* bound = dynamic_cast<Bound const*>(top); bound)
-          {
-            return *bound == static_cast<Bound const&>(*this);
-          }
-          else
-          {
-            return std::is_same_v<Bound, null>;
-          }
-        }
-        else
-        {
-          return false;
-        }
-      }
-
-      auto type() const noexcept -> std::type_info const& override
-      {
-        return typeid(Bound);
-      }
-
-      auto write(std::ostream & os) const -> std::ostream & override
-      {
-        if constexpr (is_output_streamable_v<Bound const&>)
-        {
-          return os << static_cast<Bound const&>(*this);
-        }
-        else
-        {
-          return os << magenta("#,(") << green(typeid(Bound).name()) << faint(" #;", static_cast<Bound const*>(this)) << magenta(")");
-        }
-      }
-    };
-
-  public:
-    using BasePointer<Top, Ts...>::BasePointer;
-
-    using BasePointer<Top, Ts...>::dereferenceable;
-
-    using BasePointer<Top, Ts...>::get;
-
-    template <typename Bound,
-              typename Allocator = collector::default_allocator<void>,
-              typename... Us>
-    static auto allocate(Us&&... xs)
-    {
-      if constexpr (std::is_same_v<Bound, Top>)
-      {
-        return heterogeneous_pointer(primary_collector().make<Top, Allocator>(std::forward<decltype(xs)>(xs)...));
-      }
-      else if constexpr (std::is_class_v<Bound>)
-      {
-        return heterogeneous_pointer(primary_collector().make<binder<Bound>, Allocator>(std::forward<decltype(xs)>(xs)...));
-      }
-      else
-      {
-        return heterogeneous_pointer(std::forward<decltype(xs)>(xs)...);
-      }
-    }
-
-    template <typename U>
-    inline auto as() const -> decltype(auto)
-    {
-      if constexpr (std::is_same_v<std::decay_t<U>, Top>)
-      {
-        return BasePointer<Top, Ts...>::operator *();
-      }
-      else if constexpr (std::is_class_v<std::decay_t<U>>)
-      {
-        if (auto data = dynamic_cast<std::add_pointer_t<U>>(get()); data)
-        {
-          return *data;
-        }
-        else
-        {
-          throw std::runtime_error(lexical_cast<std::string>("no viable conversion from ", demangle(type()), " to ", demangle(typeid(U))));
-        }
-      }
-      else
-      {
-        return BasePointer<Top, Ts...>::template as<U>();
-      }
-    }
-
-    template <typename U>
-    inline auto as() -> decltype(auto)
-    {
-      if constexpr (std::is_same_v<std::decay_t<U>, Top>)
-      {
-        return BasePointer<Top, Ts...>::operator *();
-      }
-      else if constexpr (std::is_class_v<std::decay_t<U>>)
-      {
-        if (auto data = dynamic_cast<std::add_pointer_t<U>>(get()); data)
-        {
-          return *data;
-        }
-        else
-        {
-          throw std::runtime_error(lexical_cast<std::string>("no viable conversion from ", demangle(type()), " to ", demangle(typeid(U))));
-        }
-      }
-      else
-      {
-        return BasePointer<Top, Ts...>::template as<U>();
-      }
-    }
-
-    template <typename U>
-    inline auto as_const() const -> decltype(auto)
-    {
-      return as<std::add_const_t<U>>();
-    }
-
-    inline auto compare(heterogeneous_pointer const& rhs) const -> bool
-    {
-      if (dereferenceable())
-      {
-        return *this ? get()->compare(rhs.get()) : rhs.is<null>();
-      }
-      else
-      {
-        return BasePointer<Top, Ts...>::compare(rhs);
-      }
-    }
-
-    template <typename U>
-    inline auto is() const
-    {
-      return type() == typeid(std::decay_t<U>);
-    }
-
-    template <typename U, REQUIRES(std::is_class<U>)>
-    inline auto is_also() const
-    {
-      return dynamic_cast<std::add_pointer_t<U>>(get()) != nullptr;
-    }
-
-    inline auto type() const -> std::type_info const&
-    {
-      if (dereferenceable())
-      {
-        return *this ? get()->type() : typeid(null);
-      }
-      else
-      {
-        return BasePointer<Top, Ts...>::type();
-      }
-    }
-
-    inline auto write(std::ostream & os) const -> std::ostream &
-    {
-      if (dereferenceable())
-      {
-        return *this ? get()->write(os) : os << magenta("()");
-      }
-      else
-      {
-        return BasePointer<Top, Ts...>::write(os);
-      }
-    }
-
-    friend auto operator <<(std::ostream & os, heterogeneous_pointer const& datum) -> std::ostream &
-    {
-      return datum.write(os);
-    }
-
-    inline auto begin()
-    {
-      return dereferenceable() and *this ? get()->begin() : typename Top::iterator();
-    }
-
-    inline auto begin() const
-    {
-      return dereferenceable() and *this ? get()->cbegin() : typename Top::const_iterator();
-    }
-
-    inline auto cbegin() const
-    {
-      return dereferenceable() and *this ? get()->cbegin() : typename Top::const_iterator();
-    }
-
-    inline auto end()
-    {
-      return dereferenceable() and *this ? get()->end() : typename Top::iterator();
-    }
-
-    inline auto end() const
-    {
-      return dereferenceable() and *this ? get()->cend() : typename Top::const_iterator();
-    }
-
-    inline auto cend() const
-    {
-      return dereferenceable() and *this ? get()->cend() : typename Top::const_iterator();
-    }
-  };
-} // namespace memory
-} // namespace meevax
-
-#endif // INCLUDED_MEEVAX_MEMORY_HETEROGENEOUS_POINTER_HPP
diff --git a/include/meevax/memory/integer_set.hpp b/include/meevax/memory/integer_set.hpp
index 1394c74f5..a9c47d222 100644
--- a/include/meevax/memory/integer_set.hpp
+++ b/include/meevax/memory/integer_set.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -17,13 +17,11 @@
 #ifndef INCLUDED_MEEVAX_MEMORY_INTEGER_SET_HPP
 #define INCLUDED_MEEVAX_MEMORY_INTEGER_SET_HPP
 
-#include <array>
 #include <cassert>
-#include <climits>
+#include <climits> // CHAR_BIT
 #include <cstdint>
 #include <iterator>
 #include <limits>
-#include <type_traits>
 
 #include <meevax/bit/bit_cast.hpp>
 #include <meevax/bit/log2.hpp>
@@ -48,10 +46,14 @@ inline namespace memory
 
     subset * data[N] = {};
 
+    std::size_t max = 0;
+
     struct const_iterator : public std::iterator<std::bidirectional_iterator_tag, T>
     {
       subset const* const* data = nullptr;
 
+      std::size_t max = 0;
+
       std::size_t i = std::numeric_limits<std::size_t>::max();
 
       typename subset::const_iterator iter;
@@ -60,6 +62,7 @@ inline namespace memory
 
       explicit const_iterator(integer_set const* container, std::size_t i, std::uintptr_t j = 0) noexcept
         : data { container->data }
+        , max  { container->max }
         , i    { i }
       {
         assert(i <= N);
@@ -68,6 +71,7 @@ inline namespace memory
 
       explicit const_iterator(integer_set const* container) noexcept
         : data { container->data }
+        , max  { container->max }
         , i    { N }
       {
         decrement_unless_truthy();
@@ -78,12 +82,21 @@ inline namespace memory
       {
         assert(data);
 
-        for (; good(); ++i, j = 0)
+        if (good())
         {
           if (data[i] and (iter = data[i]->lower_bound(j)).good())
           {
             return;
           }
+          else for (++i; i <= max; ++i)
+          {
+            if (data[i] and (iter = data[i]->lower_bound(0)).good())
+            {
+              return;
+            }
+          }
+
+          i = N;
         }
 
         iter = {};
@@ -198,21 +211,30 @@ inline namespace memory
 
     auto insert(T value)
     {
-      if (auto [key, datum] = split(value); data[key])
+      if (auto [i, j] = split(value); data[i])
       {
-        data[key]->insert(datum);
+        data[i]->insert(j);
       }
       else
       {
-        data[key] = new subset();
-        data[key]->insert(datum);
+        max = std::max(max, i);
+        data[i] = new subset();
+        data[i]->insert(j);
       }
     }
 
     auto erase(T value) noexcept
+    {
+      if (auto [i, j] = split(value); data[i])
+      {
+        data[i]->erase(j);
+      }
+    }
+
+    auto contains(T value) noexcept -> bool
     {
       auto [i, j] = split(value);
-      data[i]->erase(j);
+      return data[i] and data[i]->contains(j);
     }
 
     auto begin() const noexcept
@@ -235,6 +257,11 @@ inline namespace memory
     {
       return std::distance(begin(), end());
     }
+
+    auto swap(integer_set & other)
+    {
+      std::swap(data, other.data);
+    }
   };
 
   template <typename T, std::size_t E>
@@ -254,17 +281,23 @@ inline namespace memory
 
       auto increment_unless_truthy() noexcept
       {
-        auto i = index / 64;
-        auto j = index % 64;
-
-        for (; i < N / 64; ++i, j = 0)
+        if (auto i = index / 64; i < N/64)
         {
-          if (auto datum = data[i] & (~0ul << j); datum)
+          if (auto datum = data[i] & (~0ul << index % 64); datum)
           {
             index = i * 64 + __builtin_ctzl(datum);
             assert(data[index / 64] & (1ul << index % 64));
             return;
           }
+          else while (++i < N/64)
+          {
+            if (auto datum = data[i]; datum)
+            {
+              index = i * 64 + __builtin_ctzl(datum);
+              assert(data[index / 64] & (1ul << index % 64));
+              return;
+            }
+          }
         }
 
         index = N;
@@ -274,17 +307,23 @@ inline namespace memory
 
       auto decrement_unless_truthy() noexcept
       {
-        auto i = index / 64;
-        auto j = index % 64;
-
-        for (; i < N / 64; --i, j = 63)
+        if (auto i = index / 64; i < N/64)
         {
-          if (auto datum = data[i] & (~0ul >> (63 - j)); datum)
+          if (auto datum = data[i] & (~0ul >> (63 - index % 64)); datum)
           {
             index = i * 64 + (63 - __builtin_clzl(datum));
             assert(data[index / 64] & (1ul << index % 64));
             return;
           }
+          else while (--i < N/64)
+          {
+            if (auto datum = data[i]; datum)
+            {
+              index = i * 64 + (63 - __builtin_clzl(datum));
+              assert(data[index / 64] & (1ul << index % 64));
+              return;
+            }
+          }
         }
 
         index = N;
@@ -369,6 +408,13 @@ inline namespace memory
       data[i] &= ~(1ul << j);
     }
 
+    auto contains(T value) noexcept -> bool
+    {
+      auto i = reinterpret_cast<std::size_t>(value) / 64;
+      auto j = reinterpret_cast<std::size_t>(value) % 64;
+      return data[i] & (1ul << j);
+    }
+
     auto lower_bound(T value) const noexcept
     {
       return const_iterator(this, reinterpret_cast<std::size_t>(value));
diff --git a/include/meevax/memory/literal.hpp b/include/meevax/memory/literal.hpp
index 23fd26cb9..7649bc4d5 100644
--- a/include/meevax/memory/literal.hpp
+++ b/include/meevax/memory/literal.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/memory/marker.hpp b/include/meevax/memory/marker.hpp
deleted file mode 100644
index 766dddad1..000000000
--- a/include/meevax/memory/marker.hpp
+++ /dev/null
@@ -1,60 +0,0 @@
-/*
-   Copyright 2018-2023 Tatsuya Yamasaki.
-
-   Licensed under the Apache License, Version 2.0 (the "License");
-   you may not use this file except in compliance with the License.
-   You may obtain a copy of the License at
-
-       http://www.apache.org/licenses/LICENSE-2.0
-
-   Unless required by applicable law or agreed to in writing, software
-   distributed under the License is distributed on an "AS IS" BASIS,
-   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-   See the License for the specific language governing permissions and
-   limitations under the License.
-*/
-
-#ifndef INCLUDED_MEEVAX_MEMORY_MARKER_HPP
-#define INCLUDED_MEEVAX_MEMORY_MARKER_HPP
-
-namespace meevax
-{
-inline namespace memory
-{
-  class marker
-  {
-    static inline std::size_t phase;
-
-    std::size_t value;
-
-  public:
-    struct initializer
-    {
-      explicit initializer();
-    };
-
-    marker() noexcept
-      : value { phase }
-    {}
-
-    auto mark() noexcept
-    {
-      value = phase;
-    }
-
-    auto marked() const noexcept
-    {
-      return value == phase;
-    }
-
-    static auto clear() noexcept
-    {
-      ++phase;
-    }
-  };
-
-  static marker::initializer initializer;
-} // namespace memory
-} // namespace meevax
-
-#endif // INCLUDED_MEEVAX_MEMORY_MARKER_HPP
diff --git a/include/meevax/memory/model.hpp b/include/meevax/memory/model.hpp
index cb4228dff..889c1e94e 100644
--- a/include/meevax/memory/model.hpp
+++ b/include/meevax/memory/model.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/memory/nan_boxing_pointer.hpp b/include/meevax/memory/nan_boxing_pointer.hpp
index 9fad8ceba..13d19bdb0 100644
--- a/include/meevax/memory/nan_boxing_pointer.hpp
+++ b/include/meevax/memory/nan_boxing_pointer.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/memory/pointer_set.hpp b/include/meevax/memory/pointer_set.hpp
index b0fd052de..94ce4a2fe 100644
--- a/include/meevax/memory/pointer_set.hpp
+++ b/include/meevax/memory/pointer_set.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/memory/simple_allocator.hpp b/include/meevax/memory/simple_allocator.hpp
deleted file mode 100644
index 1c08cbc38..000000000
--- a/include/meevax/memory/simple_allocator.hpp
+++ /dev/null
@@ -1,128 +0,0 @@
-/*
-   Copyright 2018-2023 Tatsuya Yamasaki.
-
-   Licensed under the Apache License, Version 2.0 (the "License");
-   you may not use this file except in compliance with the License.
-   You may obtain a copy of the License at
-
-       http://www.apache.org/licenses/LICENSE-2.0
-
-   Unless required by applicable law or agreed to in writing, software
-   distributed under the License is distributed on an "AS IS" BASIS,
-   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-   See the License for the specific language governing permissions and
-   limitations under the License.
-*/
-
-#ifndef INCLUDED_MEEVAX_MEMORY_SIMPLE_ALLOCATOR_HPP
-#define INCLUDED_MEEVAX_MEMORY_SIMPLE_ALLOCATOR_HPP
-
-#include <algorithm> // std::max
-#include <array>
-#include <cstddef>
-#include <cstdint>
-#include <type_traits>
-#include <utility> // std::exchange
-
-namespace meevax
-{
-inline namespace memory
-{
-template <typename T, typename Capacity = std::integral_constant<std::size_t, 1024>>
-class simple_allocator
-{
-  struct alignas(T) chunk
-  {
-    chunk * next;
-  };
-
-  class chunks
-  {
-    static constexpr auto chunk_size = std::max(sizeof(T), sizeof(chunk));
-
-    std::array<std::uint8_t, chunk_size * Capacity::value> data;
-
-    std::size_t size = Capacity::value;
-
-  public:
-    chunks * const next;
-
-    explicit constexpr chunks(chunks * next = nullptr)
-      : next { next }
-    {}
-
-    ~chunks()
-    {
-      delete next;
-    }
-
-    auto remaining() const noexcept
-    {
-      return size;
-    }
-
-    auto pop()
-    {
-      return reinterpret_cast<value_type *>(std::addressof(data[chunk_size * --size]));
-    }
-  };
-
-  chunk * recycled_chunk = nullptr;
-
-  chunks * fresh_chunks;
-
-public:
-  using value_type = T;
-
-  using propagate_on_container_move_assignment = std::false_type;
-
-  using size_type = std::size_t;
-
-  using difference_type = std::ptrdiff_t;
-
-  using is_always_equal = std::false_type;
-
-  explicit simple_allocator()
-    : fresh_chunks { new chunks() }
-  {}
-
-  simple_allocator(simple_allocator &&) = delete;
-
-  simple_allocator(simple_allocator const&) = delete;
-
-  ~simple_allocator()
-  {
-    delete fresh_chunks;
-  }
-
-  auto operator =(simple_allocator &&) -> simple_allocator & = delete;
-
-  auto operator =(simple_allocator const&) -> simple_allocator & = delete;
-
-  auto allocate(size_type = 1)
-  {
-    if (recycled_chunk)
-    {
-      return reinterpret_cast<value_type *>(std::exchange(recycled_chunk, recycled_chunk->next));
-    }
-    else
-    {
-      if (not (*fresh_chunks).remaining())
-      {
-        fresh_chunks = new chunks(fresh_chunks);
-      }
-
-      return (*fresh_chunks).pop();
-    }
-  }
-
-  auto deallocate(value_type * p, size_type = 1) -> void
-  {
-    reinterpret_cast<chunk *>(p)->next = recycled_chunk;
-    recycled_chunk = reinterpret_cast<chunk *>(p);
-  }
-};
-} // namespace memory
-} // namespace meevax
-
-#endif // INCLUDED_MEEVAX_MEMORY_SIMPLE_ALLOCATOR_HPP
diff --git a/include/meevax/memory/simple_pointer.hpp b/include/meevax/memory/simple_pointer.hpp
index 650674b0c..2fa2e0d2d 100644
--- a/include/meevax/memory/simple_pointer.hpp
+++ b/include/meevax/memory/simple_pointer.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/memory/tagged_pointer.hpp b/include/meevax/memory/tagged_pointer.hpp
index a0c43156c..a3f788e38 100644
--- a/include/meevax/memory/tagged_pointer.hpp
+++ b/include/meevax/memory/tagged_pointer.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/type_traits/integer.hpp b/include/meevax/type_traits/integer.hpp
index 5fe1ffa7d..7d24bfa7c 100644
--- a/include/meevax/type_traits/integer.hpp
+++ b/include/meevax/type_traits/integer.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/type_traits/is_array_subscriptable.hpp b/include/meevax/type_traits/is_array_subscriptable.hpp
index 93f9da9ce..0a2224da8 100644
--- a/include/meevax/type_traits/is_array_subscriptable.hpp
+++ b/include/meevax/type_traits/is_array_subscriptable.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/type_traits/is_dereferenceable.hpp b/include/meevax/type_traits/is_dereferenceable.hpp
index 393aaf7ec..1ec3e18cf 100644
--- a/include/meevax/type_traits/is_dereferenceable.hpp
+++ b/include/meevax/type_traits/is_dereferenceable.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/type_traits/is_equality_comparable.hpp b/include/meevax/type_traits/is_equality_comparable.hpp
index 4702ee7da..9af7dd53c 100644
--- a/include/meevax/type_traits/is_equality_comparable.hpp
+++ b/include/meevax/type_traits/is_equality_comparable.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/type_traits/is_output_streamable.hpp b/include/meevax/type_traits/is_output_streamable.hpp
index 74dd11891..eff1212c1 100644
--- a/include/meevax/type_traits/is_output_streamable.hpp
+++ b/include/meevax/type_traits/is_output_streamable.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/type_traits/is_reference_wrapper.hpp b/include/meevax/type_traits/is_reference_wrapper.hpp
index 14a60d6db..e49419765 100644
--- a/include/meevax/type_traits/is_reference_wrapper.hpp
+++ b/include/meevax/type_traits/is_reference_wrapper.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/type_traits/requires.hpp b/include/meevax/type_traits/requires.hpp
index 96fe8db5f..24fe77313 100644
--- a/include/meevax/type_traits/requires.hpp
+++ b/include/meevax/type_traits/requires.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/utility/combination.hpp b/include/meevax/utility/combination.hpp
index 6842933b3..7908694df 100644
--- a/include/meevax/utility/combination.hpp
+++ b/include/meevax/utility/combination.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/utility/debug.hpp b/include/meevax/utility/debug.hpp
index 5ebdf551a..2d72dcb50 100644
--- a/include/meevax/utility/debug.hpp
+++ b/include/meevax/utility/debug.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/utility/demangle.hpp b/include/meevax/utility/demangle.hpp
index e8999997d..684571fd9 100644
--- a/include/meevax/utility/demangle.hpp
+++ b/include/meevax/utility/demangle.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/utility/hexdump.hpp b/include/meevax/utility/hexdump.hpp
index da7a1227e..e625262f2 100644
--- a/include/meevax/utility/hexdump.hpp
+++ b/include/meevax/utility/hexdump.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/include/meevax/utility/unwrap_reference_wrapper.hpp b/include/meevax/utility/unwrap_reference_wrapper.hpp
index 4afd986de..9578d43e3 100644
--- a/include/meevax/utility/unwrap_reference_wrapper.hpp
+++ b/include/meevax/utility/unwrap_reference_wrapper.hpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/script/benchmark.sh b/script/benchmark.sh
index 4201d0d2c..1476d5e94 100755
--- a/script/benchmark.sh
+++ b/script/benchmark.sh
@@ -21,20 +21,20 @@ quotient()
 
 tsv()
 {
-  printf "script\tMeevax\tChibi-Scheme\tGauche\n"
+  printf "\tMeevax\tChibi-Scheme\tGauche\n"
 
-  for each in $(scripts)
+  for script in $(scripts)
   do
-    t0=$(real meevax       "$root/benchmark/$each.ss")
-    t1=$(real chibi-scheme "$root/benchmark/$each.ss")
-    t2=$(real gosh         "$root/benchmark/$each.ss")
+    t0=$(real meevax       "$root/benchmark/$script.ss")
+    t1=$(real chibi-scheme "$root/benchmark/$script.ss")
+    t2=$(real gosh         "$root/benchmark/$script.ss")
 
     printf "%s\t%s\t%s\t%s\n" \
-      "$each" \
+      "$script" \
       "$t0" \
       "$t1 (x$(quotient "$t0" "$t1"))" \
       "$t2 (x$(quotient "$t0" "$t2"))"
   done
 }
 
-tsv | column -t -s"$(printf '\t')"
+tsv | column --table --separator "$(printf '\t')"
diff --git a/script/cachegrind.sh b/script/cachegrind.sh
index 14fa2c7e6..afc30341a 100755
--- a/script/cachegrind.sh
+++ b/script/cachegrind.sh
@@ -4,6 +4,6 @@ stem="/tmp/cachegrind"
 
 valgrind --tool=cachegrind \
          --cachegrind-out-file=$stem.out \
-         meevax "$@"
+         "$@"
 
-# cg_annotate /tmp/$stem.out
+cg_annotate $stem.out
diff --git a/script/massif.sh b/script/massif.sh
index 444ad4a0d..49c8a7591 100755
--- a/script/massif.sh
+++ b/script/massif.sh
@@ -7,6 +7,6 @@ valgrind --tool=massif \
          --quiet \
          --stacks=yes \
          --time-unit=B \
-         meevax "$@"
+         "$@"
 
 massif-visualizer $stem.out
diff --git a/src/chrono/duration.cpp b/src/chrono/duration.cpp
index 2c1218a5b..9a8b4f052 100644
--- a/src/chrono/duration.cpp
+++ b/src/chrono/duration.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/format.cpp b/src/format.cpp
deleted file mode 100644
index 583dcf8f6..000000000
--- a/src/format.cpp
+++ /dev/null
@@ -1,29 +0,0 @@
-/*
-   Copyright 2018-2023 Tatsuya Yamasaki.
-
-   Licensed under the Apache License, Version 2.0 (the "License");
-   you may not use this file except in compliance with the License.
-   You may obtain a copy of the License at
-
-       http://www.apache.org/licenses/LICENSE-2.0
-
-   Unless required by applicable law or agreed to in writing, software
-   distributed under the License is distributed on an "AS IS" BASIS,
-   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-   See the License for the specific language governing permissions and
-   limitations under the License.
-*/
-
-#include <meevax/kernel/input_file_port.hpp>
-
-auto main(int const, char const* const* const argv) -> int
-{
-  using namespace meevax;
-
-  for (let const& x : input_file_port(argv[1]))
-  {
-    std::cout << x << std::flush;
-  }
-
-  return EXIT_SUCCESS;
-}
diff --git a/src/kernel/binary_input_file_port.cpp b/src/kernel/binary_input_file_port.cpp
index 7172b3d7f..4d1d89eb5 100644
--- a/src/kernel/binary_input_file_port.cpp
+++ b/src/kernel/binary_input_file_port.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -64,7 +64,14 @@ inline namespace kernel
 
   auto binary_input_file_port::peek() -> object
   {
-    return get_ready() ? make<exact_integer>(ifstream.peek()) : eof_object;
+    if (get_ready())
+    {
+      return make<exact_integer>(ifstream.peek());
+    }
+    else
+    {
+      return eof_object;
+    }
   }
 
   auto operator <<(std::ostream & output, binary_input_file_port const& datum) -> std::ostream &
diff --git a/src/kernel/binary_output_file_port.cpp b/src/kernel/binary_output_file_port.cpp
index bed54bb04..2e7ff9455 100644
--- a/src/kernel/binary_output_file_port.cpp
+++ b/src/kernel/binary_output_file_port.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/boolean.cpp b/src/kernel/boolean.cpp
index b5493203d..474e49422 100644
--- a/src/kernel/boolean.cpp
+++ b/src/kernel/boolean.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/boot.cpp b/src/kernel/boot.cpp
index 33151b2b0..eb96a8e16 100644
--- a/src/kernel/boot.cpp
+++ b/src/kernel/boot.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -207,10 +207,16 @@ inline namespace kernel
         return car(xs).as<character>().property().is_lower_case();
       });
 
-      library.define<procedure>("digit-value", [](let const& xs)
+      library.define<procedure>("digit-value", [](let const& xs) -> object
       {
-        auto digit_value = car(xs).as<character>().digit_value();
-        return digit_value ? make<exact_integer>(*digit_value) : f;
+        if (auto digit_value = car(xs).as<character>().digit_value(); digit_value)
+        {
+          return make<exact_integer>(*digit_value);
+        }
+        else
+        {
+          return f;
+        }
       });
 
       library.define<procedure>("char->integer", [](let const& xs)
@@ -637,9 +643,16 @@ inline namespace kernel
         return make<exact_integer>(length(car(xs)));
       });
 
-      library.define<procedure>("length+", [](let const& xs)
+      library.define<procedure>("length+", [](let const& xs) -> object
       {
-        return is_circular_list(car(xs)) ? f : make<exact_integer>(length(car(xs)));
+        if (is_circular_list(car(xs)))
+        {
+          return f;
+        }
+        else
+        {
+          return make<exact_integer>(length(car(xs)));
+        }
       });
 
       library.define<procedure>("append", [](let const& xs)
@@ -1309,8 +1322,8 @@ inline namespace kernel
       {
         return make<procedure>(cadr(xs).as<symbol>(),
                                reinterpret_cast<primitive_procedure::signature>(
-                                 primary_collector().dlsym(cadr(xs).as<symbol>(),
-                                                           primary_collector().dlopen(car(xs).as<string>()))));
+                                 default_collector::dlsym(cadr(xs).as<symbol>(),
+                                                          default_collector::dlopen(car(xs).as<string>()))));
       });
     });
 
@@ -1749,13 +1762,13 @@ inline namespace kernel
 
       library.define<procedure>("make-syntactic-closure", [](let const& xs)
       {
-        return make<syntactic_closure>(car(xs), cadr(xs), caddr(xs));
+        return make<syntactic_closure>(car(xs), cons(cadr(xs), caddr(xs)));
       });
     });
 
     define<library>("(meevax system)", [](library & library)
     {
-      library.define<procedure>("get-environment-variable", [](let const& xs)
+      library.define<procedure>("get-environment-variable", [](let const& xs) -> object
       {
         if (auto s = std::getenv(static_cast<std::string>(car(xs).as<string>()).c_str()))
         {
@@ -1769,7 +1782,7 @@ inline namespace kernel
 
       library.define<procedure>("get-environment-variables", []()
       {
-        let alist = unit;
+        let alist = nullptr;
 
         for (auto iter = environ; *iter; ++iter)
         {
diff --git a/src/kernel/box.cpp b/src/kernel/box.cpp
index c21aadd1c..ba7624005 100644
--- a/src/kernel/box.cpp
+++ b/src/kernel/box.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/character.cpp b/src/kernel/character.cpp
index 80afce788..107b8c49f 100644
--- a/src/kernel/character.cpp
+++ b/src/kernel/character.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/closure.cpp b/src/kernel/closure.cpp
index e7005ed8f..e5c9d58a6 100644
--- a/src/kernel/closure.cpp
+++ b/src/kernel/closure.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/comparator.cpp b/src/kernel/comparator.cpp
index e58ec3c0b..60013845b 100644
--- a/src/kernel/comparator.cpp
+++ b/src/kernel/comparator.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -21,7 +21,7 @@ namespace meevax
 {
 inline namespace kernel
 {
-  auto find(object & b) -> object const&
+  auto find(object & b) -> object &
   {
     if (let & x = car(b); x.is<box>())
     {
@@ -35,14 +35,16 @@ inline namespace kernel
 
   auto union_find(object const& x, object const& y, std::unordered_map<object, object> & forest)
   {
-    using rank = std::uint32_t;
+    using rank = std::int32_t;
 
     if (auto iterator_x = forest.find(x); iterator_x != forest.end())
     {
       if (auto iterator_y = forest.find(y); iterator_y != forest.end())
       {
-        if (let root_x = find(iterator_x->second),
-                root_y = find(iterator_y->second); eq(root_x, root_y))
+        auto&& root_x = find(iterator_x->second);
+        auto&& root_y = find(iterator_y->second);
+
+        if (eq(root_x, root_y))
         {
           return true;
         }
diff --git a/src/kernel/complex.cpp b/src/kernel/complex.cpp
index 4e535f82d..194beb6a5 100644
--- a/src/kernel/complex.cpp
+++ b/src/kernel/complex.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/continuation.cpp b/src/kernel/continuation.cpp
index af565863d..49cdda9d2 100644
--- a/src/kernel/continuation.cpp
+++ b/src/kernel/continuation.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/environment.cpp b/src/kernel/environment.cpp
index ef1a4acc9..44eb22d1a 100644
--- a/src/kernel/environment.cpp
+++ b/src/kernel/environment.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -17,6 +17,7 @@
 #include <meevax/kernel/environment.hpp>
 #include <meevax/kernel/input_file_port.hpp>
 #include <meevax/kernel/library.hpp>
+#include <meevax/kernel/optimizer.hpp>
 
 namespace meevax
 {
@@ -74,9 +75,9 @@ inline namespace kernel
     */
     if (s or e or c)
     {
-      d = cons(std::exchange(s, unit),
-               std::exchange(e, unit),
-               std::exchange(c, unit), d);
+      d = cons(std::exchange(s, nullptr),
+               std::exchange(e, nullptr),
+               std::exchange(c, nullptr), d);
     }
 
     let const result = execute(optimize(compile(expression)));
@@ -96,7 +97,7 @@ inline namespace kernel
     if (x.is_also<error>())
     {
       x.as<error>().raise(); // NOTE: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84476
-      return unit;
+      return nullptr;
     }
     else
     {
@@ -184,7 +185,7 @@ inline namespace kernel
       {
         return [=](let const& renamings)
         {
-          return map([&](let const& identity)
+          return map([&](let const& identity) -> object
                      {
                        assert(identity.is<absolute>());
                        assert(car(identity).is_also<identifier>());
@@ -222,7 +223,7 @@ inline namespace kernel
     {
       assert(immigrant.is<absolute>());
 
-      if (let const& inhabitant = std::as_const(*this).identify(car(immigrant), unit, unit); inhabitant == f or interactive)
+      if (let const& inhabitant = std::as_const(*this).identify(car(immigrant), nullptr, nullptr); inhabitant == f or interactive)
       {
         second = cons(immigrant, second);
       }
@@ -256,8 +257,6 @@ inline namespace kernel
 
   template struct configurator<environment>;
 
-  template struct dynamic_environment<environment>;
-
   template struct syntactic_environment<environment>;
 } // namespace kernel
 } // namespace meevax
diff --git a/src/kernel/eof.cpp b/src/kernel/eof.cpp
index 9e22d3812..3d94b1220 100644
--- a/src/kernel/eof.cpp
+++ b/src/kernel/eof.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/error.cpp b/src/kernel/error.cpp
index 001e8bfdc..622688962 100644
--- a/src/kernel/error.cpp
+++ b/src/kernel/error.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/exact_integer.cpp b/src/kernel/exact_integer.cpp
index 85ab62834..3c498d050 100644
--- a/src/kernel/exact_integer.cpp
+++ b/src/kernel/exact_integer.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/ghost.cpp b/src/kernel/ghost.cpp
index 397cdf159..eefff637c 100644
--- a/src/kernel/ghost.cpp
+++ b/src/kernel/ghost.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/identity.cpp b/src/kernel/identity.cpp
index 1f35a3efb..df5a8df5c 100644
--- a/src/kernel/identity.cpp
+++ b/src/kernel/identity.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/implementation_dependent.cpp b/src/kernel/implementation_dependent.cpp
index acaa6eedb..4694f6a8d 100644
--- a/src/kernel/implementation_dependent.cpp
+++ b/src/kernel/implementation_dependent.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/include.cpp b/src/kernel/include.cpp
index 5f4acd139..df0e0485b 100644
--- a/src/kernel/include.cpp
+++ b/src/kernel/include.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/input_file_port.cpp b/src/kernel/input_file_port.cpp
index 623278094..0731f42be 100644
--- a/src/kernel/input_file_port.cpp
+++ b/src/kernel/input_file_port.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/input_string_port.cpp b/src/kernel/input_string_port.cpp
index be509ed21..6cf0383d8 100644
--- a/src/kernel/input_string_port.cpp
+++ b/src/kernel/input_string_port.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/instruction.cpp b/src/kernel/instruction.cpp
index f2cb1ada6..e2e1d9dc3 100644
--- a/src/kernel/instruction.cpp
+++ b/src/kernel/instruction.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/interaction_environment.cpp b/src/kernel/interaction_environment.cpp
index 140b57610..f1ce7ed85 100644
--- a/src/kernel/interaction_environment.cpp
+++ b/src/kernel/interaction_environment.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/library.cpp b/src/kernel/library.cpp
index e60b02995..282418bff 100644
--- a/src/kernel/library.cpp
+++ b/src/kernel/library.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -68,7 +68,7 @@ inline namespace kernel
 
   auto library::resolve() -> object
   {
-    if (let const unresolved_declarations = std::exchange(declarations, unit); unresolved_declarations.is<pair>())
+    if (let const unresolved_declarations = std::exchange(declarations, nullptr); unresolved_declarations.is<pair>())
     {
       for (let const& unresolved_declaration : unresolved_declarations)
       {
@@ -76,7 +76,7 @@ inline namespace kernel
       }
     }
 
-    return map([this](let const& export_spec)
+    return map([this](let const& export_spec) -> object
                {
                  if (export_spec.is<pair>())
                  {
@@ -85,12 +85,12 @@ inline namespace kernel
                    assert(cadr(export_spec).is_also<identifier>());
                    assert(caddr(export_spec).is_also<identifier>());
                    return make<absolute>(caddr(export_spec),
-                                         cdr(identify(cadr(export_spec), unit, unit)));
+                                         cdr(identify(cadr(export_spec), nullptr, nullptr)));
                  }
                  else
                  {
                    assert(export_spec.is_also<identifier>());
-                   return identify(export_spec, unit, unit);
+                   return identify(export_spec, nullptr, nullptr);
                  }
                },
                export_specs);
diff --git a/src/kernel/list.cpp b/src/kernel/list.cpp
index 2789fc490..4b35dd936 100644
--- a/src/kernel/list.cpp
+++ b/src/kernel/list.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -29,7 +29,7 @@ inline namespace kernel
     }
     else
     {
-      return unit;
+      return nullptr;
     }
   }
 
@@ -41,7 +41,7 @@ inline namespace kernel
     }
     else
     {
-      return unit;
+      return nullptr;
     }
   }
 
@@ -51,8 +51,8 @@ inline namespace kernel
     {
       if (let const& x1 = cdr(x0); x1.is<pair>())
       {
-        let const& x2 = cdr(x1),
-                   y1 = cdr(y0);
+        let const& x2 = cdr(x1);
+        let const& y1 = cdr(y0);
 
         return not eq(x2, y1) and is_list(x2, y1);
       }
@@ -78,8 +78,8 @@ inline namespace kernel
     {
       if (let const& x1 = cdr(x0); x1.is<pair>())
       {
-        let const& x2 = cdr(x1),
-                   y1 = cdr(y0);
+        let const& x2 = cdr(x1);
+        let const& y1 = cdr(y0);
 
         return eq(x2, y1) or is_circular_list(x2, y1);
       }
@@ -105,8 +105,8 @@ inline namespace kernel
     {
       if (let const& x1 = cdr(x0); x1.is<pair>())
       {
-        let const& x2 = cdr(x1),
-                   y1 = cdr(y0);
+        let const& x2 = cdr(x1);
+        let const& y1 = cdr(y0);
 
         return not eq(x2, y1) and is_dotted_list(x2, y1);
       }
@@ -146,7 +146,7 @@ inline namespace kernel
     }
     else
     {
-      return unit;
+      return nullptr;
     }
   }
 
@@ -154,12 +154,12 @@ inline namespace kernel
   {
     if (0 < k)
     {
-      cdr(drop(x, k - 1)) = unit;
+      cdr(drop(x, k - 1)) = nullptr;
       return x;
     }
     else
     {
-      return unit;
+      return nullptr;
     }
   }
 
@@ -212,7 +212,7 @@ inline namespace kernel
     }
     else
     {
-      return unit;
+      return nullptr;
     }
   }
 
@@ -229,24 +229,24 @@ inline namespace kernel
     }
     else
     {
-      cdr(x) = unit;
+      cdr(x) = nullptr;
     }
   }
 
   auto drop_right(object & x, std::size_t k) -> object
   {
-    if (let const y = drop(x, k); y.is<pair>())
+    if (let const& y = drop(x, k); y.is<pair>())
     {
       drop_right(x, cdr(y));
       return x;
     }
     else
     {
-      return unit;
+      return nullptr;
     }
   }
 
-  auto length(object const& x) -> std::size_t
+  auto length(object const& x) -> std::ptrdiff_t
   {
     return std::distance(x.begin(), x.end());
   }
@@ -300,11 +300,7 @@ inline namespace kernel
     }
     else
     {
-      let const cdr_x = cdr(x);
-
-      cdr(x) = y;
-
-      return append_reverse(cdr_x, x);
+      return append_reverse(std::exchange(cdr(x), y), x);
     }
   }
 
@@ -322,7 +318,7 @@ inline namespace kernel
 
   auto reverse(object const& xs) -> object
   {
-    return reverse(xs, unit);
+    return reverse(xs, nullptr);
   }
 
   auto reverse(object & xs, object const& a) -> object
@@ -333,17 +329,13 @@ inline namespace kernel
     }
     else
     {
-      let tail = cdr(xs);
-
-      cdr(xs) = a;
-
-      return reverse(tail, xs);
+      return reverse(std::exchange(cdr(xs), a), xs);
     }
   }
 
   auto reverse(object & xs) -> object
   {
-    return reverse(xs, unit);
+    return reverse(xs, nullptr);
   }
 
   auto memq(object const& x, object const& xs) -> object const&
diff --git a/src/kernel/number.cpp b/src/kernel/number.cpp
index c723d3e93..bc7748271 100644
--- a/src/kernel/number.cpp
+++ b/src/kernel/number.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -21,6 +21,7 @@
 #include <meevax/kernel/ghost.hpp>
 #include <meevax/kernel/number.hpp>
 #include <meevax/kernel/string.hpp>
+#include <meevax/utility/combination.hpp>
 
 namespace meevax
 {
diff --git a/src/kernel/optimizer.cpp b/src/kernel/optimizer.cpp
new file mode 100644
index 000000000..daa39b23b
--- /dev/null
+++ b/src/kernel/optimizer.cpp
@@ -0,0 +1,218 @@
+/*
+   Copyright 2018-2024 Tatsuya Yamasaki.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+#include <meevax/kernel/list.hpp>
+#include <meevax/kernel/optimizer.hpp>
+
+namespace meevax
+{
+inline namespace kernel
+{
+  struct analysis
+  {
+    static constexpr auto n = 23;
+
+    std::size_t adjacencies[n][n] = {};
+
+    ~analysis()
+    {
+      constexpr std::size_t width = 18;
+
+      std::cerr << std::string(width, ' ');
+
+      for (auto i = 0; i < n; ++i)
+      {
+        std::cerr << std::setw(width) << std::right << static_cast<instruction>(i);
+      }
+
+      std::cerr << std::endl;
+
+      for (auto i = 0; i < n; ++i)
+      {
+        std::cerr << std::setw(width) << std::right << static_cast<instruction>(i);
+
+        for (auto j = 0; j < n; ++j)
+        {
+          std::cerr << std::setw(width) << std::right << adjacencies[i][j];
+        }
+
+        std::cerr << std::endl;
+      }
+    }
+
+    auto adjacency(object const& i, object const& j) -> decltype(auto)
+    {
+      return adjacencies[static_cast<std::underlying_type_t<instruction>>(i.as<instruction>())]
+                        [static_cast<std::underlying_type_t<instruction>>(j.as<instruction>())];
+    }
+  };
+
+  auto analyze(object const& c) -> void
+  {
+    assert(c.is<pair>());
+
+    assert(car(c).is<instruction>());
+
+    static auto a = analysis();
+
+    switch (car(c).as<instruction>())
+    {
+    case instruction::join:
+    case instruction::tail_call:
+    case instruction::tail_letrec:
+    case instruction::return_:
+    case instruction::stop:
+      assert(cdr(c).is<null>());
+      break;
+
+    case instruction::call:
+    case instruction::cons:
+    case instruction::drop:
+    case instruction::dummy:
+    case instruction::letrec:
+      analyze(cdr(c));
+      a.adjacency(car(c), cadr(c))++;
+      break;
+
+    case instruction::current:
+    case instruction::install:
+    case instruction::load_absolute:
+    case instruction::load_constant:
+    case instruction::load_relative:
+    case instruction::load_variadic:
+    case instruction::store_absolute:
+    case instruction::store_relative:
+    case instruction::store_variadic:
+      analyze(cddr(c));
+      a.adjacency(car(c), caddr(c))++;
+      break;
+
+    case instruction::load_closure:
+    case instruction::load_continuation:
+      analyze(cadr(c));
+      analyze(cddr(c));
+      a.adjacency(car(c), caddr(c))++;
+      break;
+
+    case instruction::select:
+      analyze(cadr(c));
+      analyze(caddr(c));
+      analyze(cdddr(c));
+      a.adjacency(car(c), cadddr(c))++;
+      break;
+
+    case instruction::tail_select:
+      assert(cdddr(c).is<null>());
+      analyze(cadr(c));
+      analyze(caddr(c));
+      break;
+    }
+  }
+
+  auto merge_constants(object & c) -> void
+  {
+    assert(c.is<pair>());
+
+    assert(car(c).is<instruction>());
+
+    switch (car(c).as<instruction>())
+    {
+    case instruction::load_constant: /* ----------------------------------------
+    *
+    *  (load-constant x
+    *   load-constant y
+    *   cons
+    *   ...)
+    *
+    *  => (load-constant (y . x)
+    *      ...)
+    *
+    * ----------------------------------------------------------------------- */
+      if (cddr(c).is<pair>() and
+          caddr(c).is<instruction>() and caddr(c).as<instruction>() == instruction::load_constant and
+          cddddr(c).is<pair>() and
+          caddddr(c).is<instruction>() and caddddr(c).as<instruction>() == instruction::cons)
+      {
+        cadr(c) = cons(cadddr(c), cadr(c));
+        cddr(c) = cdddddr(c);
+        merge_constants(c);
+      }
+      else
+      {
+        merge_constants(cddr(c));
+      }
+      break;
+
+    case instruction::join:
+    case instruction::tail_call:
+    case instruction::tail_letrec:
+    case instruction::return_:
+    case instruction::stop:
+      assert(cdr(c).is<null>());
+      break;
+
+    case instruction::call:
+    case instruction::cons:
+    case instruction::drop:
+    case instruction::dummy:
+    case instruction::letrec:
+      merge_constants(cdr(c));
+      break;
+
+    case instruction::current:
+    case instruction::install:
+    case instruction::load_absolute:
+    case instruction::load_relative:
+    case instruction::load_variadic:
+    case instruction::store_absolute:
+    case instruction::store_relative:
+    case instruction::store_variadic:
+      merge_constants(cddr(c));
+      break;
+
+    case instruction::load_closure:
+    case instruction::load_continuation:
+      merge_constants(cadr(c));
+      merge_constants(cddr(c));
+      break;
+
+    case instruction::select:
+      merge_constants(cadr(c));
+      merge_constants(caddr(c));
+      merge_constants(cdddr(c));
+      break;
+
+    case instruction::tail_select:
+      assert(cdddr(c).is<null>());
+      merge_constants(cadr(c));
+      merge_constants(caddr(c));
+      break;
+    }
+  }
+
+  auto optimize(object code) -> object
+  {
+    merge_constants(code);
+
+    if constexpr (false)
+    {
+      analyze(code);
+    }
+
+    return code;
+  }
+} // namespace kernel
+} // namespace meevax
diff --git a/src/kernel/output_file_port.cpp b/src/kernel/output_file_port.cpp
index 2c80334fe..04a3f10f3 100644
--- a/src/kernel/output_file_port.cpp
+++ b/src/kernel/output_file_port.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/output_string_port.cpp b/src/kernel/output_string_port.cpp
index ac384b930..90b0bdec7 100644
--- a/src/kernel/output_string_port.cpp
+++ b/src/kernel/output_string_port.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/pair.cpp b/src/kernel/pair.cpp
index 3d8ca0305..8e0e83dfa 100644
--- a/src/kernel/pair.cpp
+++ b/src/kernel/pair.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -20,19 +20,13 @@ namespace meevax
 {
 inline namespace kernel
 {
-  let unit { nullptr };
+  let unit = nullptr;
 
-  pair::pair(object const& a)
-    : std::pair<object, object> { a, unit }
-  {}
-
-  pair::pair(object const& a, object const& b)
-    : std::pair<object, object> { a, b }
-  {}
-
-  auto pair::compare(pair const* that) const -> bool
+  auto pair::compare(top const* other) const -> bool
   {
-    return that and *this == *that;
+    auto pare = dynamic_cast<pair const*>(other);
+
+    return pare and *this == *pare;
   }
 
   auto pair::type() const noexcept -> std::type_info const&
@@ -64,7 +58,7 @@ inline namespace kernel
     {
       os << magenta("(") << car(datum);
 
-      for (let xs = cdr(datum); xs != unit; xs = cdr(xs))
+      for (let xs = cdr(datum); not xs.is<null>(); xs = cdr(xs))
       {
         if (xs.is<pair>())
         {
diff --git a/src/kernel/procedure.cpp b/src/kernel/procedure.cpp
index ad3ebe09d..1329373cf 100644
--- a/src/kernel/procedure.cpp
+++ b/src/kernel/procedure.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/ratio.cpp b/src/kernel/ratio.cpp
index bb299af19..b6405a4ab 100644
--- a/src/kernel/ratio.cpp
+++ b/src/kernel/ratio.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/standard_error_port.cpp b/src/kernel/standard_error_port.cpp
index aa868062e..c5c7eabb8 100644
--- a/src/kernel/standard_error_port.cpp
+++ b/src/kernel/standard_error_port.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/standard_input_port.cpp b/src/kernel/standard_input_port.cpp
index 05b267b89..3c217d5a1 100644
--- a/src/kernel/standard_input_port.cpp
+++ b/src/kernel/standard_input_port.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/standard_output_port.cpp b/src/kernel/standard_output_port.cpp
index 3736bb72f..dfd7cecfb 100644
--- a/src/kernel/standard_output_port.cpp
+++ b/src/kernel/standard_output_port.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/string.cpp b/src/kernel/string.cpp
index 8a6cc8f87..dfb03f95c 100644
--- a/src/kernel/string.cpp
+++ b/src/kernel/string.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/symbol.cpp b/src/kernel/symbol.cpp
index 0426dad78..866659fc8 100644
--- a/src/kernel/symbol.cpp
+++ b/src/kernel/symbol.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/textual_input_port.cpp b/src/kernel/textual_input_port.cpp
index 54e811a0a..cbe8246cd 100644
--- a/src/kernel/textual_input_port.cpp
+++ b/src/kernel/textual_input_port.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -285,7 +285,7 @@ inline namespace kernel
               }
               else
               {
-                return unit;
+                return nullptr;
               }
             }
             else
@@ -408,7 +408,7 @@ inline namespace kernel
         }
         catch (std::integral_constant<char, ')'> const&)
         {
-          return unit;
+          return nullptr;
         }
         catch (std::integral_constant<char, '.'> const&)
         {
diff --git a/src/kernel/textual_output_port.cpp b/src/kernel/textual_output_port.cpp
index bf2d26bbf..34cb91722 100644
--- a/src/kernel/textual_output_port.cpp
+++ b/src/kernel/textual_output_port.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/transformer.cpp b/src/kernel/transformer.cpp
index 293435886..b6e41ee79 100644
--- a/src/kernel/transformer.cpp
+++ b/src/kernel/transformer.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/kernel/vector.cpp b/src/kernel/vector.cpp
index f38946924..d8fa11faa 100644
--- a/src/kernel/vector.cpp
+++ b/src/kernel/vector.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/main.cpp b/src/main.cpp
index 2dfdc45b7..217d7a0c7 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/memory/collector.cpp b/src/memory/collector.cpp
deleted file mode 100644
index 9c5413ba0..000000000
--- a/src/memory/collector.cpp
+++ /dev/null
@@ -1,171 +0,0 @@
-/*
-   Copyright 2018-2023 Tatsuya Yamasaki.
-
-   Licensed under the Apache License, Version 2.0 (the "License");
-   you may not use this file except in compliance with the License.
-   You may obtain a copy of the License at
-
-       http://www.apache.org/licenses/LICENSE-2.0
-
-   Unless required by applicable law or agreed to in writing, software
-   distributed under the License is distributed on an "AS IS" BASIS,
-   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-   See the License for the specific language governing permissions and
-   limitations under the License.
-*/
-
-#if __unix__
-#include <dlfcn.h> // dlopen, dlclose, dlerror
-#else
-#error
-#endif
-
-#include <iostream>
-
-#include <meevax/memory/collector.hpp>
-#include <meevax/memory/literal.hpp>
-
-namespace meevax
-{
-inline namespace memory
-{
-  collector::~collector()
-  {
-    clear();
-  }
-
-  auto collector::clear() -> void
-  {
-    for (auto&& tag : tags)
-    {
-      delete tag;
-      tags.erase(tag);
-    }
-  }
-
-  auto collector::collect() -> void
-  {
-    allocation = 0;
-
-    return mark(), sweep();
-  }
-
-  auto collector::count() noexcept -> std::size_t
-  {
-    return std::size(tags);
-  }
-
-  auto collector::dlclose(void * const handle) -> void
-  {
-    if (handle and ::dlclose(handle))
-    {
-      std::cerr << ::dlerror() << std::endl;
-    }
-  }
-
-  auto collector::dlopen(std::string const& filename) -> void *
-  {
-    ::dlerror(); // Clear
-
-    try
-    {
-      return dynamic_linked_libraries.at(filename).get();
-    }
-    catch (std::out_of_range const&)
-    {
-      if (auto handle = ::dlopen(filename.c_str(), RTLD_LAZY | RTLD_GLOBAL); handle)
-      {
-        dynamic_linked_libraries.emplace(std::piecewise_construct,
-                                         std::forward_as_tuple(filename),
-                                         std::forward_as_tuple(handle, dlclose));
-
-        return dlopen(filename);
-      }
-      else
-      {
-        throw std::runtime_error(::dlerror());
-      }
-    }
-  }
-
-  auto collector::dlsym(std::string const& symbol, void * const handle) -> void *
-  {
-    if (auto address = ::dlsym(handle, symbol.c_str()); address)
-    {
-      return address;
-    }
-    else
-    {
-      throw std::runtime_error(::dlerror());
-    }
-  }
-
-  auto collector::mark() -> void
-  {
-    marker::clear();
-
-    auto is_root_object = [begin = tags.begin()](mutator * given)
-    {
-      /*
-         If the given mutator is a non-root object, then an object containing
-         this mutator as a data member exists somewhere in memory.
-
-         Containing the mutator as a data member means that the address of the
-         mutator is contained in the interval of the object's base-address ~
-         base-address + object-size. The tag is present to keep track of the
-         base-address and size of the object needed here.
-      */
-      auto iter = tags.lower_bound(reinterpret_cast<tag *>(given));
-
-      return iter == begin or not (*--iter)->contains(given);
-    };
-
-    for (auto&& mutator : mutators)
-    {
-      assert(mutator);
-      assert(mutator->object);
-
-      if (not mutator->object->marked() and is_root_object(mutator))
-      {
-        mark(mutator->object);
-      }
-    }
-  }
-
-  auto collector::mark(tag * const tag) -> void
-  {
-    assert(tag);
-
-    if (not tag->marked())
-    {
-      tag->mark();
-
-      const auto lower_address = tag->lower_address<mutator>();
-      const auto upper_address = tag->upper_address<mutator>();
-
-      for (auto iter = mutators.lower_bound(lower_address); iter != mutators.end() and *iter < upper_address; ++iter)
-      {
-        mark((*iter)->object);
-      }
-    }
-  }
-
-  auto collector::sweep() -> void
-  {
-    for (auto&& tag : tags)
-    {
-      if (not tag->marked())
-      {
-        delete tag;
-        tags.erase(tag);
-      }
-    }
-  }
-
-  auto primary_collector() -> collector &
-  {
-    static auto c = collector();
-    return c;
-  }
-} // namespace memory
-} // namespace meevax
diff --git a/src/memory/marker.cpp b/src/memory/marker.cpp
deleted file mode 100644
index a95f35b1b..000000000
--- a/src/memory/marker.cpp
+++ /dev/null
@@ -1,35 +0,0 @@
-/*
-   Copyright 2018-2023 Tatsuya Yamasaki.
-
-   Licensed under the Apache License, Version 2.0 (the "License");
-   you may not use this file except in compliance with the License.
-   You may obtain a copy of the License at
-
-       http://www.apache.org/licenses/LICENSE-2.0
-
-   Unless required by applicable law or agreed to in writing, software
-   distributed under the License is distributed on an "AS IS" BASIS,
-   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-   See the License for the specific language governing permissions and
-   limitations under the License.
-*/
-
-#include <cstdint>
-
-#include <meevax/memory/marker.hpp>
-
-namespace meevax
-{
-inline namespace memory
-{
-  std::size_t count = 0;
-
-  marker::initializer::initializer()
-  {
-    if (not count++)
-    {
-      phase = {};
-    }
-  }
-} // namespace memory
-} // namespace meevax
diff --git a/src/memory/model.cpp b/src/memory/model.cpp
index 68629b50f..99b903677 100644
--- a/src/memory/model.cpp
+++ b/src/memory/model.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/src/utility/demangle.cpp b/src/utility/demangle.cpp
index 13bc5c846..d686c8063 100644
--- a/src/utility/demangle.cpp
+++ b/src/utility/demangle.cpp
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018-2023 Tatsuya Yamasaki.
+   Copyright 2018-2024 Tatsuya Yamasaki.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
diff --git a/test/allocator.cpp b/test/allocator.cpp
index 658b12d80..3c6cafc2c 100644
--- a/test/allocator.cpp
+++ b/test/allocator.cpp
@@ -6,7 +6,7 @@
 
 #include <meevax/chrono/duration.hpp>
 #include <meevax/kernel/pair.hpp>
-#include <meevax/memory/simple_allocator.hpp>
+#include <meevax/memory/allocator.hpp>
 #include <meevax/utility/debug.hpp>
 #include <meevax/utility/demangle.hpp>
 
@@ -62,15 +62,15 @@ struct hoge
 auto main() -> int
 {
   std::cout << measure<int, std::allocator<int>>() << std::endl;
-  std::cout << measure<int, meevax::simple_allocator<int>>() << std::endl;
+  std::cout << measure<int, meevax::allocator<int>>() << std::endl;
 
-  std::cout << measure<meevax::pair, meevax::simple_allocator<meevax::pair>>() << std::endl;
+  std::cout << measure<meevax::pair, meevax::allocator<meevax::pair>>() << std::endl;
 
-  std::cout << measure<std::array<std::uint8_t,  1>, meevax::simple_allocator<std::array<std::uint8_t,  1>>>() << std::endl;
-  std::cout << measure<std::array<std::uint8_t,  2>, meevax::simple_allocator<std::array<std::uint8_t,  2>>>() << std::endl;
-  std::cout << measure<std::array<std::uint8_t,  4>, meevax::simple_allocator<std::array<std::uint8_t,  4>>>() << std::endl;
-  std::cout << measure<std::array<std::uint8_t,  8>, meevax::simple_allocator<std::array<std::uint8_t,  8>>>() << std::endl;
-  std::cout << measure<std::array<std::uint8_t, 16>, meevax::simple_allocator<std::array<std::uint8_t, 16>>>() << std::endl;
+  std::cout << measure<std::array<std::uint8_t,  1>, meevax::allocator<std::array<std::uint8_t,  1>>>() << std::endl;
+  std::cout << measure<std::array<std::uint8_t,  2>, meevax::allocator<std::array<std::uint8_t,  2>>>() << std::endl;
+  std::cout << measure<std::array<std::uint8_t,  4>, meevax::allocator<std::array<std::uint8_t,  4>>>() << std::endl;
+  std::cout << measure<std::array<std::uint8_t,  8>, meevax::allocator<std::array<std::uint8_t,  8>>>() << std::endl;
+  std::cout << measure<std::array<std::uint8_t, 16>, meevax::allocator<std::array<std::uint8_t, 16>>>() << std::endl;
 
   return EXIT_SUCCESS;
 }
diff --git a/test/collector.cpp b/test/collector.cpp
index e5143348e..6e69055df 100644
--- a/test/collector.cpp
+++ b/test/collector.cpp
@@ -7,7 +7,7 @@ auto main() -> int
 {
   using namespace meevax;
 
-  const auto gc_count = primary_collector().count();
+  const auto gc_count = default_collector::count();
 
   // scope
   {
@@ -16,19 +16,19 @@ auto main() -> int
       let y = make<symbol>("y");
       let z = make<symbol>("z");
 
-      assert(primary_collector().count() == gc_count + 3);
-      primary_collector().collect();
-      assert(primary_collector().count() == gc_count + 3);
+      assert(default_collector::count() == gc_count + 3);
+      default_collector::collect();
+      assert(default_collector::count() == gc_count + 3);
     }
 
-    assert(primary_collector().count() == gc_count + 3);
-    primary_collector().collect();
-    assert(primary_collector().count() == gc_count);
+    assert(default_collector::count() == gc_count + 3);
+    default_collector::collect();
+    assert(default_collector::count() == gc_count);
   }
 
-  assert(primary_collector().count() == gc_count);
-  primary_collector().collect();
-  assert(primary_collector().count() == gc_count);
+  assert(default_collector::count() == gc_count);
+  default_collector::collect();
+  assert(default_collector::count() == gc_count);
 
   // copy
   {
@@ -42,9 +42,9 @@ auto main() -> int
     assert(eqv(x, y));
   }
 
-  assert(primary_collector().count() == gc_count + 1);
-  primary_collector().collect();
-  assert(primary_collector().count() == gc_count);
+  assert(default_collector::count() == gc_count + 1);
+  default_collector::collect();
+  assert(default_collector::count() == gc_count);
 
   // move
   {
@@ -63,17 +63,17 @@ auto main() -> int
     assert(x.is<symbol>());
     assert(x.as<symbol>() == "x");
 
-    assert(primary_collector().count() == gc_count + 1);
-    primary_collector().collect();
-    assert(primary_collector().count() == gc_count + 1);
+    assert(default_collector::count() == gc_count + 1);
+    default_collector::collect();
+    assert(default_collector::count() == gc_count + 1);
 
     assert(x.is<symbol>());
     assert(x.as<symbol>() == "x");
   }
 
-  assert(primary_collector().count() == gc_count + 1);
-  primary_collector().collect();
-  assert(primary_collector().count() == gc_count);
+  assert(default_collector::count() == gc_count + 1);
+  default_collector::collect();
+  assert(default_collector::count() == gc_count);
 
   // proper list
   {
@@ -91,9 +91,9 @@ auto main() -> int
       assert(y.as<symbol>() == "y");
       assert(z.as<symbol>() == "z");
 
-      assert(primary_collector().count() == gc_count + 3);
-      primary_collector().collect();
-      assert(primary_collector().count() == gc_count + 3);
+      assert(default_collector::count() == gc_count + 3);
+      default_collector::collect();
+      assert(default_collector::count() == gc_count + 3);
 
       return list(x, y, z);
     };
@@ -106,9 +106,9 @@ auto main() -> int
     assert(cadr(a).is<symbol>());
     assert(caddr(a).is<symbol>());
 
-    assert(primary_collector().count() == gc_count + 6);
-    primary_collector().collect();
-    assert(primary_collector().count() == gc_count + 6);
+    assert(default_collector::count() == gc_count + 6);
+    default_collector::collect();
+    assert(default_collector::count() == gc_count + 6);
 
     assert(length(a) == 3);
 
@@ -117,9 +117,9 @@ auto main() -> int
     assert(caddr(a).is<symbol>());
   }
 
-  assert(primary_collector().count() == gc_count + 6);
-  primary_collector().collect();
-  assert(primary_collector().count() == gc_count);
+  assert(default_collector::count() == gc_count + 6);
+  default_collector::collect();
+  assert(default_collector::count() == gc_count);
 
   // improper list
   {
@@ -137,7 +137,7 @@ auto main() -> int
       assert(b.as<symbol>() == "b");
       assert(c.as<symbol>() == "c");
 
-      assert(primary_collector().count() == gc_count + 3);
+      assert(default_collector::count() == gc_count + 3);
 
       return circular_list(a, b, c);
     };
@@ -149,7 +149,7 @@ auto main() -> int
     assert(caddr(x).as<symbol>() == "c");
     assert(cadddr(x).as<symbol>() == "a");
 
-    primary_collector().collect();
+    default_collector::collect();
 
     assert(car(x).as<symbol>() == "a");
     assert(cadr(x).as<symbol>() == "b");
@@ -157,28 +157,28 @@ auto main() -> int
     assert(cadddr(x).as<symbol>() == "a");
   }
 
-  primary_collector().collect();
+  default_collector::collect();
 
   // change class
   {
     let x = make<symbol>("hoge");
 
-    primary_collector().collect();
+    default_collector::collect();
 
-    assert(primary_collector().count() == gc_count + 1);
+    assert(default_collector::count() == gc_count + 1);
     assert(x.is<symbol>());
     assert(x.as<symbol>() == "hoge");
 
     x = make<exact_integer>(42);
 
-    primary_collector().collect();
+    default_collector::collect();
 
-    assert(primary_collector().count() == gc_count + 1);
+    assert(default_collector::count() == gc_count + 1);
     assert(x.is<exact_integer>());
     assert(x.as<exact_integer>() == 42);
   }
 
-  primary_collector().collect();
+  default_collector::collect();
 
   // number
   {
@@ -194,22 +194,22 @@ auto main() -> int
       assert(y.as<double>() == 42);
     }
 
-    assert(primary_collector().count() == gc_count);
-    primary_collector().collect();
-    assert(primary_collector().count() == gc_count);
+    assert(default_collector::count() == gc_count);
+    default_collector::collect();
+    assert(default_collector::count() == gc_count);
   }
 
-  primary_collector().collect();
+  default_collector::collect();
 
   // read list
   {
-    const auto gc_count = primary_collector().count();
+    const auto gc_count = default_collector::count();
 
     input_string_port("(a a a)").read();
 
-    primary_collector().collect();
+    default_collector::collect();
 
-    assert(primary_collector().count() == gc_count + 1);
+    assert(default_collector::count() == gc_count + 1);
   }
 
   return EXIT_SUCCESS;
diff --git a/test/environment.cpp b/test/environment.cpp
index a0b2b51ff..3c59cea9b 100644
--- a/test/environment.cpp
+++ b/test/environment.cpp
@@ -21,7 +21,7 @@ auto main() -> int
     assert(interaction_environment().is<environment>());
   }
 
-  const auto gc_count = primary_collector().count();
+  const auto gc_count = default_collector::count();
 
   assert(gc_count == specials_count);
 
@@ -38,10 +38,9 @@ auto main() -> int
 
   assert(libraries().empty());
 
-  primary_collector().collect();
-  primary_collector().collect(); // for vector type
+  default_collector::collect();
 
-  assert(gc_count - 1 == primary_collector().count()); // -1 is interaction_environment
+  assert(gc_count - 1 == default_collector::count()); // -1 is interaction_environment
 
   return EXIT_SUCCESS;
 }
diff --git a/test/list.cpp b/test/list.cpp
index c67f8911a..3d848fe7b 100644
--- a/test/list.cpp
+++ b/test/list.cpp
@@ -12,7 +12,7 @@ auto main() -> int
       c = make<symbol>("c"),
       d = make<symbol>("d");
 
-  assert(lexical_cast<std::string>(cons(a, unit)) == "(a)");
+  assert(lexical_cast<std::string>(cons(a, nullptr)) == "(a)");
 
   assert(lexical_cast<std::string>(cons(list(a), list(b, c, d))) == "((a) b c d)");
 
@@ -24,7 +24,9 @@ auto main() -> int
 
   assert(lexical_cast<std::string>(list(a, make<exact_integer>(3 + 4), c)) == "(a 7 c)");
 
-  assert(lexical_cast<std::string>(list()) == "()");
+  assert(lexical_cast<std::string>(list()) == "nullptr");
+
+  assert(lexical_cast<std::string>(object(list())) == "()");
 
   assert(lexical_cast<std::string>(xcons(list(b, c), a)) == "(a b c)");
 
diff --git a/test/nan_boxing_pointer.cpp b/test/nan_boxing_pointer.cpp
index aeef00482..d4fadf233 100644
--- a/test/nan_boxing_pointer.cpp
+++ b/test/nan_boxing_pointer.cpp
@@ -130,29 +130,29 @@ auto main() -> int
   }
 
   {
-    let x = make<std::uint32_t>(static_cast<std::uint32_t>(1)),
-        y = make<std::uint32_t>(static_cast<std::uint32_t>(2));
+    let x = make(static_cast<std::int32_t>(1)),
+        y = make(static_cast<std::int32_t>(2));
 
     // PRINT(demangle(x.type()));
     // PRINT(demangle(y.type()));
 
-    assert(x.is<std::uint32_t>());
-    assert(y.is<std::uint32_t>());
+    assert(x.is<std::int32_t>());
+    assert(y.is<std::int32_t>());
 
     let const pare = cons(x, y);
 
     PRINT(demangle(pare.as<pair>().first.type()));
     PRINT(demangle(pare.as<pair>().second.type()));
 
-    assert(pare.as<pair>().first.is<std::uint32_t>());
-    assert(pare.as<pair>().first.as<std::uint32_t>() == 1);
-    assert(pare.as<pair>().second.is<std::uint32_t>());
-    assert(pare.as<pair>().second.as<std::uint32_t>() == 2);
+    assert(pare.as<pair>().first.is<std::int32_t>());
+    assert(pare.as<pair>().first.as<std::int32_t>() == 1);
+    assert(pare.as<pair>().second.is<std::int32_t>());
+    assert(pare.as<pair>().second.as<std::int32_t>() == 2);
 
-    assert(car(pare).is<std::uint32_t>());
-    assert(car(pare).as<std::uint32_t>() == 1);
-    assert(cdr(pare).is<std::uint32_t>());
-    assert(cdr(pare).as<std::uint32_t>() == 2);
+    assert(car(pare).is<std::int32_t>());
+    assert(car(pare).as<std::int32_t>() == 1);
+    assert(cdr(pare).is<std::int32_t>());
+    assert(cdr(pare).as<std::int32_t>() == 2);
   }
 
   delete p;
diff --git a/test/pair.cpp b/test/pair.cpp
index 5d7508263..c2f07004c 100644
--- a/test/pair.cpp
+++ b/test/pair.cpp
@@ -2,7 +2,6 @@
 
 #include <cassert>
 #include <meevax/kernel/environment.hpp>
-#include <meevax/memory/simple_allocator.hpp>
 
 auto main() -> int
 {
@@ -12,15 +11,5 @@ auto main() -> int
 
   assert((make<pair, std::allocator<void>>().is<pair>()));
 
-  assert((make<pair, meevax::simple_allocator<void>>().is<pair>()));
-
-  /*
-     Without this explicit call to primary_collector().clear(), illegal memory
-     access by the destructor of the collector will occur for objects allocated
-     by allocators other than std::allocator. See the comments attached to
-     collector::tagged::allocator for a detailed description of this problem.
-  */
-  primary_collector().clear();
-
   return EXIT_SUCCESS;
 }
diff --git a/test/vector.cpp b/test/vector.cpp
index ebf9772c0..c77622f30 100644
--- a/test/vector.cpp
+++ b/test/vector.cpp
@@ -8,45 +8,45 @@ auto main() -> int
 {
   using namespace meevax;
 
-  const auto gc_count = primary_collector().count();
+  const auto gc_count = default_collector::count();
 
   // make-vector
   {
     let const v = make<vector>(list(make<symbol>("a"),
                                     make<symbol>("b"),
                                     make<symbol>("c")));
-    primary_collector().collect();
+    default_collector::collect();
 
     assert(v.is<vector>());
     assert(v.as<vector>().vector.size() == 3);
-    assert(primary_collector().count() == gc_count + 4);
+    assert(default_collector::count() == gc_count + 4);
 
-    primary_collector().collect();
+    default_collector::collect();
 
     assert(v.is<vector>());
     assert(v.as<vector>().vector.size() == 3);
-    assert(primary_collector().count() == gc_count + 4);
+    assert(default_collector::count() == gc_count + 4);
 
-    primary_collector().collect();
+    default_collector::collect();
 
     assert(v.is<vector>());
     assert(v.as<vector>().vector.size() == 3);
-    assert(primary_collector().count() == gc_count + 4);
+    assert(default_collector::count() == gc_count + 4);
 
     v.as<vector>().vector.clear();
 
     assert(v.is<vector>());
     assert(v.as<vector>().vector.size() == 0);
-    assert(primary_collector().count() == gc_count + 4);
+    assert(default_collector::count() == gc_count + 4);
 
-    primary_collector().collect();
+    default_collector::collect();
 
     assert(v.is<vector>());
     assert(v.as<vector>().vector.size() == 0);
-    assert(primary_collector().count() == gc_count + 1);
+    assert(default_collector::count() == gc_count + 1);
   }
 
-  primary_collector().collect();
+  default_collector::collect();
 
   // list->vector
   {
@@ -56,38 +56,38 @@ auto main() -> int
 
     assert(v.is<vector>());
     assert(v.as<vector>().vector.size() == 3);
-    assert(primary_collector().count() == gc_count + 7);
+    assert(default_collector::count() == gc_count + 7);
 
-    primary_collector().collect();
+    default_collector::collect();
 
     assert(v.is<vector>());
     assert(v.as<vector>().vector.size() == 3);
-    assert(primary_collector().count() == gc_count + 4);
+    assert(default_collector::count() == gc_count + 4);
 
-    primary_collector().collect();
+    default_collector::collect();
 
     assert(v.is<vector>());
     assert(v.as<vector>().vector.size() == 3);
-    assert(primary_collector().count() == gc_count + 4);
+    assert(default_collector::count() == gc_count + 4);
 
     v.as<vector>().vector.clear();
 
     assert(v.is<vector>());
     assert(v.as<vector>().vector.size() == 0);
-    assert(primary_collector().count() == gc_count + 4);
+    assert(default_collector::count() == gc_count + 4);
 
-    primary_collector().collect();
+    default_collector::collect();
 
     assert(v.is<vector>());
     assert(v.as<vector>().vector.size() == 0);
-    assert(primary_collector().count() == gc_count + 1);
+    assert(default_collector::count() == gc_count + 1);
   }
 
-  primary_collector().collect();
+  default_collector::collect();
 
   // list->vector
   {
-    auto const gc_count = primary_collector().count();
+    auto const gc_count = default_collector::count();
 
     let const v = make<vector>(input_string_port("(a b c)").read());
 
@@ -97,31 +97,31 @@ auto main() -> int
     assert(v.as<vector>().vector[1].as<symbol>() == "b");
     assert(v.as<vector>().vector[2].as<symbol>() == "c");
 
-    primary_collector().collect();
+    default_collector::collect();
 
     assert(v.is<vector>());
     assert(v.as<vector>().vector.size() == 3);
     assert(v.as<vector>().vector[0].as<symbol>() == "a");
     assert(v.as<vector>().vector[1].as<symbol>() == "b");
     assert(v.as<vector>().vector[2].as<symbol>() == "c");
-    assert(primary_collector().count() == gc_count + 4);
+    assert(default_collector::count() == gc_count + 4);
 
-    primary_collector().collect();
+    default_collector::collect();
 
     assert(v.is<vector>());
     assert(v.as<vector>().vector.size() == 3);
     assert(v.as<vector>().vector[0].as<symbol>() == "a");
     assert(v.as<vector>().vector[1].as<symbol>() == "b");
     assert(v.as<vector>().vector[2].as<symbol>() == "c");
-    assert(primary_collector().count() == gc_count + 4);
+    assert(default_collector::count() == gc_count + 4);
 
     v.as<vector>().vector.clear();
 
     assert(v.is<vector>());
     assert(v.as<vector>().vector.size() == 0);
-    assert(primary_collector().count() == gc_count + 4);
+    assert(default_collector::count() == gc_count + 4);
 
-    primary_collector().collect();
+    default_collector::collect();
 
     assert(v.is<vector>());
     assert(v.as<vector>().vector.size() == 0);
@@ -129,45 +129,45 @@ auto main() -> int
 
   symbols().clear();
 
-  primary_collector().collect();
+  default_collector::collect();
 
   // vector literal
   {
-    auto const gc_count = primary_collector().count();
+    auto const gc_count = default_collector::count();
 
     let const v = input_string_port("#(a b c)").read();
 
     assert(v.is<vector>());
     assert(v.as<vector>().vector.size() == 3);
 
-    primary_collector().collect();
+    default_collector::collect();
 
     assert(v.is<vector>());
     assert(v.as<vector>().vector.size() == 3);
-    assert(primary_collector().count() == gc_count + 4);
+    assert(default_collector::count() == gc_count + 4);
 
-    primary_collector().collect();
+    default_collector::collect();
 
     assert(v.is<vector>());
     assert(v.as<vector>().vector.size() == 3);
-    assert(primary_collector().count() == gc_count + 4);
+    assert(default_collector::count() == gc_count + 4);
 
     v.as<vector>().vector.clear();
 
     assert(v.is<vector>());
     assert(v.as<vector>().vector.size() == 0);
-    assert(primary_collector().count() == gc_count + 4);
+    assert(default_collector::count() == gc_count + 4);
 
-    primary_collector().collect();
+    default_collector::collect();
 
     assert(v.is<vector>());
     assert(v.as<vector>().vector.size() == 0);
-    assert(primary_collector().count() == gc_count + 4);
+    assert(default_collector::count() == gc_count + 4);
   }
 
   symbols().clear();
 
-  primary_collector().collect();
+  default_collector::collect();
 
   // vector constructor
   {