diff --git a/lib/lrama/grammar/binding.rb b/lib/lrama/grammar/binding.rb
index 5e6e7c59..718c3699 100644
--- a/lib/lrama/grammar/binding.rb
+++ b/lib/lrama/grammar/binding.rb
@@ -17,6 +17,10 @@ def resolve_symbol(symbol)
         if symbol.is_a?(Lexer::Token::InstantiateRule)
           resolved_args = symbol.args.map { |arg| resolve_symbol(arg) }
           Lrama::Lexer::Token::InstantiateRule.new(s_value: symbol.s_value, location: symbol.location, args: resolved_args, lhs_tag: symbol.lhs_tag)
+        elsif symbol.is_a?(Lexer::Token::ControlSyntax)
+          resolved = symbol.dup
+          resolved.condition = @parameter_to_arg[symbol.condition_value]
+          resolved
         else
           parameter_to_arg(symbol) || symbol
         end
diff --git a/lib/lrama/grammar/parameterizing_rule/rhs.rb b/lib/lrama/grammar/parameterizing_rule/rhs.rb
index d574841d..a5c6ef35 100644
--- a/lib/lrama/grammar/parameterizing_rule/rhs.rb
+++ b/lib/lrama/grammar/parameterizing_rule/rhs.rb
@@ -4,12 +4,20 @@ module Lrama
   class Grammar
     class ParameterizingRule
       class Rhs
-        attr_accessor :symbols, :user_code, :precedence_sym
+        attr_accessor :symbols, :user_code, :precedence_sym, :if_clause
 
         def initialize
           @symbols = []
           @user_code = nil
           @precedence_sym = nil
+          @if_clause = nil
+        end
+
+        def skip?(bindings)
+          return false unless @if_clause
+
+          resolved = bindings.resolve_symbol(@if_clause)
+          resolved.is_a?(Lexer::Token::ControlSyntax) && resolved.if? && resolved.false?
         end
 
         def resolve_user_code(bindings)
diff --git a/lib/lrama/grammar/rule_builder.rb b/lib/lrama/grammar/rule_builder.rb
index f6df22b6..e2784e38 100644
--- a/lib/lrama/grammar/rule_builder.rb
+++ b/lib/lrama/grammar/rule_builder.rb
@@ -140,12 +140,14 @@ def process_rhs
             if (created_lhs = @parameterizing_rule_resolver.created_lhs(lhs_s_value))
               @replaced_rhs << created_lhs
             else
+              next if parameterizing_rule.rhs_list.all? { |r| r.skip?(bindings) }
               lhs_token = Lrama::Lexer::Token::Ident.new(s_value: lhs_s_value, location: token.location)
               @replaced_rhs << lhs_token
               @parameterizing_rule_resolver.created_lhs_list << lhs_token
               parameterizing_rule.rhs_list.each do |r|
                 rule_builder = RuleBuilder.new(@rule_counter, @midrule_action_counter, @parameterizing_rule_resolver, lhs_tag: token.lhs_tag || parameterizing_rule.tag)
                 rule_builder.lhs = lhs_token
+                next if r.skip?(bindings)
                 r.symbols.each { |sym| rule_builder.add_rhs(bindings.resolve_symbol(sym)) }
                 rule_builder.line = line
                 rule_builder.precedence_sym = r.precedence_sym
@@ -168,6 +170,8 @@ def process_rhs
             rule_builder.setup_rules
 
             @rule_builders_for_derived_rules << rule_builder
+          when Lrama::Lexer::Token::ControlSyntax
+            # NOP
           else
             raise "Unexpected token. #{token}"
           end
diff --git a/lib/lrama/lexer.rb b/lib/lrama/lexer.rb
index 0a8f9433..82f4e765 100644
--- a/lib/lrama/lexer.rb
+++ b/lib/lrama/lexer.rb
@@ -41,6 +41,9 @@ class Lexer
       %no-stdlib
       %inline
       %locations
+      %if
+      %true
+      %false
     ).freeze
 
     def initialize(grammar_file)
diff --git a/lib/lrama/lexer/token.rb b/lib/lrama/lexer/token.rb
index 45a097f6..ed78ec62 100644
--- a/lib/lrama/lexer/token.rb
+++ b/lib/lrama/lexer/token.rb
@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require_relative 'token/char'
+require_relative 'token/control_syntax'
 require_relative 'token/ident'
 require_relative 'token/instantiate_rule'
 require_relative 'token/tag'
diff --git a/lib/lrama/lexer/token/control_syntax.rb b/lib/lrama/lexer/token/control_syntax.rb
new file mode 100644
index 00000000..16ff6c3a
--- /dev/null
+++ b/lib/lrama/lexer/token/control_syntax.rb
@@ -0,0 +1,30 @@
+module Lrama
+  class Lexer
+    class Token
+      class ControlSyntax < Token
+        attr_accessor :condition
+
+        def initialize(s_value:, location:, condition: nil)
+          @condition = condition
+          super(s_value: s_value, location: location)
+        end
+
+        def if?
+          s_value == '%if'
+        end
+
+        def true?
+          !!@condition&.s_value
+        end
+
+        def false?
+          !true?
+        end
+
+        def condition_value
+          @condition&.s_value
+        end
+      end
+    end
+  end
+end
diff --git a/lib/lrama/parser.rb b/lib/lrama/parser.rb
index 6a35dba2..1507395b 100644
--- a/lib/lrama/parser.rb
+++ b/lib/lrama/parser.rb
@@ -654,7 +654,7 @@ def token_to_str(t)
 module Lrama
   class Parser < Racc::Parser
 
-module_eval(<<'...end parser.y/module_eval...', 'parser.y', 417)
+module_eval(<<'...end parser.y/module_eval...', 'parser.y', 435)
 
 include Lrama::Report::Duration
 
@@ -728,297 +728,322 @@ def raise_parse_error(error_message, location)
 ##### State transition tables begin ###
 
 racc_action_table = [
-    94,    48,    95,   166,    48,    77,   172,    48,    77,   166,
-    48,    77,   172,    48,    77,    47,     6,    85,    69,    48,
-    48,    47,    47,    77,    74,    81,    48,    48,    47,    47,
-    40,    81,    81,    48,    41,    47,    92,    48,    81,    47,
-    44,    77,   103,   168,   169,    45,   175,   169,    96,   168,
-   169,    52,   175,   169,    86,    20,    24,    25,    26,    27,
-    28,    29,    30,    31,    32,    33,    34,    35,    36,    37,
-    38,    92,    48,     4,    47,     5,    77,   103,    48,    48,
-    47,    47,    77,   103,   116,    48,     4,    47,     5,    77,
+    94,     6,    95,    48,    85,   201,    48,    77,   211,    48,
+    77,   211,    48,    77,    47,    48,    77,   211,    48,    77,
+    47,    48,    77,    47,    40,   148,    69,    48,   149,   168,
+    48,    77,   175,    48,    77,   168,    48,    77,   175,  -118,
+    77,    86,     4,    41,     5,   183,   184,   185,     4,    44,
+     5,    96,   206,   207,   173,   206,   207,   149,   206,   207,
+    45,   206,   207,    92,   206,   207,    52,   206,   207,   170,
+   171,   172,   178,    52,   172,   170,   171,   172,   178,    92,
+   172,    52,    20,    24,    25,    26,    27,    28,    29,    30,
+    31,    32,    33,    34,    35,    36,    37,    38,    20,    24,
+    25,    26,    27,    28,    29,    30,    31,    32,    33,    34,
+    35,    36,    37,    38,    11,    12,    13,    14,    15,    16,
+    17,    18,    19,    55,    20,    24,    25,    26,    27,    28,
+    29,    30,    31,    32,    33,    34,    35,    36,    37,    38,
+    11,    12,    13,    14,    15,    16,    17,    18,    19,    43,
     20,    24,    25,    26,    27,    28,    29,    30,    31,    32,
-    33,    34,    35,    36,    37,    38,    11,    12,    13,    14,
-    15,    16,    17,    18,    19,    52,    20,    24,    25,    26,
-    27,    28,    29,    30,    31,    32,    33,    34,    35,    36,
-    37,    38,    11,    12,    13,    14,    15,    16,    17,    18,
-    19,    43,    20,    24,    25,    26,    27,    28,    29,    30,
-    31,    32,    33,    34,    35,    36,    37,    38,    48,    48,
-    47,    47,    77,   103,    48,    48,    47,    47,    77,    77,
-    48,    48,    47,    47,    77,    77,    48,    48,   196,   196,
-    77,    77,    48,    48,    47,   196,    77,    77,   148,   170,
-    52,   149,   149,   180,   181,   182,   131,   180,   181,   182,
-   131,   203,   208,   215,   204,   204,   204,    48,    48,    47,
-    47,    48,    48,    47,    47,    48,    48,    47,    47,   180,
-   181,   182,   119,   120,    55,    52,    52,    52,    52,    52,
-    61,    62,    63,    64,    65,    87,    52,    52,   106,   109,
-   111,   118,   125,   126,   128,   131,   132,    77,   140,   141,
-   142,   143,   145,   146,   153,   140,   155,   153,   159,   160,
-   161,   163,   164,   171,   176,   153,   183,   131,   187,   153,
-   189,   131,   153,   198,   153,   131,   161,   164,   205,   164,
-   161,   161,   213,   131,   161 ]
+    33,    34,    35,    36,    37,    38,    48,    48,    47,    47,
+    77,    74,    81,    48,    48,    47,    47,    52,    81,    81,
+    48,    48,    47,    47,    48,    81,    47,    52,    77,   103,
+    48,    48,    47,    47,    77,   103,    48,    48,    47,    47,
+    77,   103,   116,    48,    48,    47,    47,    77,   103,   183,
+   184,   185,   119,   120,   131,   183,   184,   185,    52,    48,
+   131,    47,    48,    77,    47,    48,    77,    47,    48,    77,
+    47,    48,    77,    47,   223,    77,   235,   224,   236,   224,
+    48,   224,    47,    48,    48,    47,    47,    48,    52,    47,
+   183,   184,   185,    52,    61,    62,    63,    64,    65,    87,
+    52,    52,   106,   109,   111,   118,   125,   126,   128,   131,
+   132,    77,   140,   141,   142,   143,   145,   146,   153,   140,
+   155,   153,   159,   160,   161,   163,   164,   174,   179,   153,
+   186,   131,   189,   191,   153,   193,   131,   153,   208,   209,
+   153,   131,   161,   164,   216,   217,   219,   220,   164,   222,
+   161,   161,  -116,   231,   131,  -118,   161,  -117 ]
 
 racc_action_check = [
-    46,   151,    46,   151,   157,   151,   157,   177,   157,   177,
-   188,   177,   188,    32,   188,    32,     1,    38,    32,    33,
-    34,    33,    34,    33,    33,    34,    35,    36,    35,    36,
-     5,    35,    36,    37,     6,    37,    44,    58,    37,    58,
-     9,    58,    58,   151,   151,    11,   157,   157,    46,   177,
-   177,    13,   188,   188,    38,    44,    44,    44,    44,    44,
-    44,    44,    44,    44,    44,    44,    44,    44,    44,    44,
-    44,    88,    59,     0,    59,     0,    59,    59,    60,    79,
-    60,    79,    60,    60,    79,    72,     2,    72,     2,    72,
+    46,     1,    46,   186,    38,   186,   193,   186,   193,   217,
+   193,   217,   219,   217,   219,   222,   219,   222,   224,   222,
+   224,    32,   224,    32,     5,   139,    32,   151,   139,   151,
+   157,   151,   157,   180,   157,   180,   192,   180,   192,   202,
+   192,    38,     0,     6,     0,   202,   202,   202,     2,     9,
+     2,    46,   186,   186,   154,   193,   193,   154,   217,   217,
+    11,   219,   219,    44,   222,   222,    13,   224,   224,   151,
+   151,   151,   157,    14,   157,   180,   180,   180,   192,    88,
+   192,    15,    44,    44,    44,    44,    44,    44,    44,    44,
+    44,    44,    44,    44,    44,    44,    44,    44,    88,    88,
     88,    88,    88,    88,    88,    88,    88,    88,    88,    88,
-    88,    88,    88,    88,    88,    88,     3,     3,     3,     3,
-     3,     3,     3,     3,     3,    14,     3,     3,     3,     3,
+    88,    88,    88,    88,     3,     3,     3,     3,     3,     3,
+     3,     3,     3,    16,     3,     3,     3,     3,     3,     3,
      3,     3,     3,     3,     3,     3,     3,     3,     3,     3,
-     3,     3,     8,     8,     8,     8,     8,     8,     8,     8,
      8,     8,     8,     8,     8,     8,     8,     8,     8,     8,
-     8,     8,     8,     8,     8,     8,     8,     8,   100,    12,
-   100,    12,   100,   100,    74,   111,    74,   111,    74,   111,
-   168,   175,   168,   175,   168,   175,   183,   189,   183,   189,
-   183,   189,   204,   205,   204,   205,   204,   205,   139,   154,
-    15,   139,   154,   165,   165,   165,   165,   173,   173,   173,
-   173,   195,   200,   212,   195,   200,   212,    67,    69,    67,
-    69,    81,   106,    81,   106,   114,   116,   114,   116,   197,
-   197,   197,    86,    86,    16,    17,    20,    24,    25,    26,
-    27,    28,    29,    30,    31,    39,    50,    55,    66,    70,
-    71,    85,    89,    90,    91,    92,    98,   110,   118,   119,
-   120,   121,   130,   131,   141,   142,   144,   145,   146,   147,
-   148,   149,   150,   156,   162,   164,   166,   167,   170,   171,
-   172,   174,   176,   186,   187,   191,   192,   194,   196,   199,
-   201,   203,   207,   208,   214 ]
+     8,     8,     8,     8,     8,     8,     8,     8,     8,     8,
+     8,     8,     8,     8,     8,     8,    33,    34,    33,    34,
+    33,    33,    34,    35,    36,    35,    36,    17,    35,    36,
+    37,    12,    37,    12,    58,    37,    58,    20,    58,    58,
+    59,    67,    59,    67,    59,    59,    60,    79,    60,    79,
+    60,    60,    79,   100,    69,   100,    69,   100,   100,   167,
+   167,   167,    86,    86,   167,   176,   176,   176,    24,    72,
+   176,    72,    74,    72,    74,   111,    74,   111,   170,   111,
+   170,   178,   170,   178,   212,   178,   228,   212,   232,   228,
+    81,   232,    81,   106,   114,   106,   114,   116,    25,   116,
+   213,   213,   213,    26,    27,    28,    29,    30,    31,    39,
+    50,    55,    66,    70,    71,    85,    89,    90,    91,    92,
+    98,   110,   118,   119,   120,   121,   130,   131,   141,   142,
+   144,   145,   146,   147,   148,   149,   150,   156,   162,   164,
+   168,   169,   171,   173,   174,   175,   177,   179,   189,   190,
+   191,   195,   196,   198,   200,   201,   203,   208,   210,   211,
+   214,   216,   218,   221,   223,   229,   233,   235 ]
 
 racc_action_pointer = [
-    63,    16,    76,    93,   nil,    23,    34,   nil,   119,    31,
-   nil,    39,   156,     5,    69,   144,   219,   179,   nil,   nil,
-   180,   nil,   nil,   nil,   181,   182,   183,   225,   226,   227,
-   228,   229,    10,    16,    17,    23,    24,    30,    12,   233,
-   nil,   nil,   nil,   nil,    32,   nil,    -5,   nil,   nil,   nil,
-   190,   nil,   nil,   nil,   nil,   191,   nil,   nil,    34,    69,
-    75,   nil,   nil,   nil,   nil,   nil,   230,   204,   nil,   205,
-   233,   232,    82,   nil,   161,   nil,   nil,   nil,   nil,    76,
-   nil,   208,   nil,   nil,   nil,   202,   218,   nil,    67,   233,
-   221,   222,   194,   nil,   nil,   nil,   nil,   nil,   244,   nil,
-   155,   nil,   nil,   nil,   nil,   nil,   209,   nil,   nil,   nil,
-   240,   162,   nil,   nil,   212,   nil,   213,   nil,   243,   208,
-   211,   240,   nil,   nil,   nil,   nil,   nil,   nil,   nil,   nil,
-   211,   248,   nil,   nil,   nil,   nil,   nil,   nil,   nil,   148,
-   nil,   200,   250,   nil,   254,   203,   206,   212,   252,   256,
-   218,    -2,   nil,   nil,   149,   nil,   219,     1,   nil,   nil,
-   nil,   nil,   223,   nil,   211,   145,   227,   216,   167,   nil,
-   227,   215,   231,   149,   220,   168,   218,     4,   nil,   nil,
-   nil,   nil,   nil,   173,   nil,   nil,   271,   220,     7,   174,
-   nil,   224,   268,   nil,   233,   161,   239,   171,   nil,   235,
-   162,   272,   nil,   273,   179,   180,   nil,   235,   232,   nil,
-   nil,   nil,   163,   nil,   276,   nil,   nil ]
+    32,     1,    38,   101,   nil,    17,    43,   nil,   127,    40,
+   nil,    54,   178,    19,    26,    34,   118,   130,   nil,   nil,
+   140,   nil,   nil,   nil,   171,   201,   206,   249,   250,   251,
+   252,   253,    18,   163,   164,   170,   171,   177,    -1,   257,
+   nil,   nil,   nil,   nil,    59,   nil,    -5,   nil,   nil,   nil,
+   213,   nil,   nil,   nil,   nil,   214,   nil,   nil,   181,   187,
+   193,   nil,   nil,   nil,   nil,   nil,   254,   188,   nil,   201,
+   257,   256,   216,   nil,   219,   nil,   nil,   nil,   nil,   194,
+   nil,   237,   nil,   nil,   nil,   226,   208,   nil,    75,   257,
+   245,   246,   215,   nil,   nil,   nil,   nil,   nil,   268,   nil,
+   200,   nil,   nil,   nil,   nil,   nil,   240,   nil,   nil,   nil,
+   264,   222,   nil,   nil,   241,   nil,   244,   nil,   267,   232,
+   235,   264,   nil,   nil,   nil,   nil,   nil,   nil,   nil,   nil,
+   235,   272,   nil,   nil,   nil,   nil,   nil,   nil,   nil,   -15,
+   nil,   221,   274,   nil,   278,   224,   227,   235,   276,   280,
+   242,    24,   nil,   nil,    14,   nil,   243,    27,   nil,   nil,
+   nil,   nil,   247,   nil,   232,   nil,   nil,   160,   251,   237,
+   225,   253,   nil,   252,   237,   256,   166,   242,   228,   240,
+    30,   nil,   nil,   nil,   nil,   nil,     0,   nil,   nil,   293,
+   297,   243,    33,     3,   nil,   247,   294,   nil,   259,   nil,
+   264,   266,    -4,   263,   nil,   nil,   nil,   nil,   267,   nil,
+   264,   270,   194,   201,   302,   nil,   303,     6,   269,     9,
+   nil,   265,    12,   260,    15,   nil,   nil,   nil,   196,   272,
+   nil,   nil,   198,   308,   nil,   274,   nil,   nil ]
 
 racc_action_default = [
-    -1,  -127,    -1,    -3,   -10,  -127,  -127,    -2,    -3,  -127,
-   -16,  -127,  -127,  -127,  -127,  -127,  -127,  -127,   -24,   -25,
-  -127,   -30,   -31,   -32,  -127,  -127,  -127,  -127,  -127,  -127,
-  -127,  -127,  -127,  -127,  -127,  -127,  -127,  -127,  -127,  -127,
-   -13,   217,    -4,   -26,  -127,   -17,  -118,   -89,   -90,  -117,
-   -14,   -19,   -81,   -20,   -21,  -127,   -23,   -29,  -127,  -127,
-  -127,   -36,   -37,   -38,   -39,   -40,   -41,   -47,   -49,  -127,
-   -52,   -42,   -74,   -76,  -127,   -79,   -80,  -126,   -43,   -84,
-   -86,  -127,   -44,   -45,   -46,  -127,  -127,   -11,    -5,    -7,
-   -91,  -127,   -64,   -18,  -119,  -120,  -121,   -15,  -127,   -22,
-   -27,   -33,  -122,  -123,   -34,   -35,  -127,   -48,   -50,   -53,
-   -72,  -127,   -75,   -77,   -84,   -85,  -127,   -87,  -127,  -127,
-  -127,  -127,    -6,    -8,    -9,  -115,   -92,   -93,   -94,   -65,
-  -127,  -127,   -82,   -28,   -51,   -54,   -73,   -78,   -88,  -127,
-   -60,  -124,  -127,   -12,  -127,  -124,  -127,  -127,   -55,  -127,
-   -58,   -62,   -66,  -125,  -127,  -116,   -95,   -96,   -98,  -114,
-   -83,   -56,  -127,   -61,  -124,   -64,   -89,   -64,  -127,  -111,
-  -127,  -124,   -89,   -64,   -64,  -127,  -124,   -63,   -67,   -68,
-  -104,  -105,  -106,  -127,   -70,   -71,  -127,  -124,   -97,  -127,
-   -99,   -64,   -55,  -103,   -57,  -127,   -89,  -107,  -112,   -59,
-  -127,   -55,  -102,   -55,  -127,  -127,  -109,  -127,   -64,  -100,
-   -69,  -108,  -127,  -113,   -55,  -110,  -101 ]
+    -1,  -138,    -1,    -3,   -10,  -138,  -138,    -2,    -3,  -138,
+   -16,  -138,  -138,  -138,  -138,  -138,  -138,  -138,   -24,   -25,
+  -138,   -30,   -31,   -32,  -138,  -138,  -138,  -138,  -138,  -138,
+  -138,  -138,  -138,  -138,  -138,  -138,  -138,  -138,  -138,  -138,
+   -13,   238,    -4,   -26,  -138,   -17,  -129,   -92,   -93,  -128,
+   -14,   -19,   -84,   -20,   -21,  -138,   -23,   -29,  -138,  -138,
+  -138,   -36,   -37,   -38,   -39,   -40,   -41,   -47,   -49,  -138,
+   -52,   -42,   -77,   -79,  -138,   -82,   -83,  -137,   -43,   -87,
+   -89,  -138,   -44,   -45,   -46,  -138,  -138,   -11,    -5,    -7,
+   -94,  -138,   -66,   -18,  -130,  -131,  -132,   -15,  -138,   -22,
+   -27,   -33,  -133,  -134,   -34,   -35,  -138,   -48,   -50,   -53,
+   -75,  -138,   -78,   -80,   -87,   -88,  -138,   -90,  -138,  -138,
+  -138,  -138,    -6,    -8,    -9,  -126,   -95,   -96,   -97,   -67,
+  -138,  -138,   -85,   -28,   -51,   -54,   -76,   -81,   -91,  -138,
+   -60,  -135,  -138,   -12,  -138,  -135,  -138,  -138,   -55,  -138,
+   -58,   -62,   -68,  -136,  -138,  -127,   -98,   -99,  -101,  -125,
+   -86,   -56,  -138,   -61,  -135,   -63,   -64,   -66,   -92,   -66,
+  -138,  -138,  -122,  -138,  -135,   -92,   -66,   -66,  -138,  -135,
+   -62,   -69,   -70,  -107,  -108,  -109,  -138,   -72,   -73,  -138,
+  -138,  -135,  -100,  -138,  -102,   -66,   -55,  -106,   -57,   -65,
+  -138,   -92,  -110,  -138,  -114,  -119,  -120,  -121,  -138,  -123,
+   -59,   -92,  -138,  -118,   -55,  -105,   -55,  -138,  -112,  -138,
+   -74,  -138,  -138,   -66,  -138,  -116,  -103,   -71,  -138,  -111,
+  -115,  -124,  -138,   -55,  -118,  -113,  -117,  -104 ]
 
 racc_goto_table = [
-   102,   102,   102,    49,   130,    73,    68,    89,   162,    91,
-   150,   139,   101,   104,   105,     1,     9,   174,    51,    53,
-    54,    42,   115,    70,   117,    79,    79,    79,    79,    56,
-   123,     3,    57,     7,   124,   154,    58,    59,    60,   179,
-   158,   107,   102,   108,   112,   194,   113,   191,   174,   157,
-    39,   122,   202,    91,   133,    97,   199,   115,    70,   138,
-    70,   209,   195,   210,   121,    46,   158,    99,   200,    93,
-   114,   206,   114,    66,   216,   188,    71,   178,   135,   184,
-   134,   110,   177,   137,   212,   190,   192,    78,    82,    83,
-    84,   136,    98,   165,   147,   127,   156,    70,   186,   173,
-   207,   144,   nil,   201,   nil,   114,   nil,   114,   nil,   nil,
-   185,   nil,   nil,   nil,   nil,   nil,   nil,   193,   nil,   165,
-   214,   nil,   nil,   nil,   nil,   197,   nil,   nil,   nil,   nil,
-   173,   197,   nil,   nil,   nil,   nil,   nil,   nil,   nil,   nil,
-   nil,   nil,   nil,   nil,   nil,   nil,   211,   197 ]
+   102,   102,   102,    49,    68,   130,    73,   162,   182,    89,
+     1,   150,    91,    51,    53,    54,   177,   195,   123,   166,
+   101,   104,   105,    70,   124,    79,    79,    79,    79,    56,
+    39,   115,    57,   117,   158,   157,    58,    59,    60,   107,
+   121,   108,   102,   218,     3,   112,     7,   113,   199,   198,
+    97,   177,   203,   122,   225,   215,    91,   139,    70,   212,
+    70,   210,   133,   158,   192,    46,   115,    99,   138,     9,
+   114,   230,   114,   226,    42,   227,   230,    93,   134,    66,
+   181,   154,   187,   228,   137,    71,   135,   110,   232,   194,
+   196,   180,   237,   167,   200,   136,    98,    70,   147,   176,
+    78,    82,    83,    84,   127,   114,   156,   114,   214,   190,
+   221,   144,   188,   nil,   nil,   nil,   nil,   nil,   nil,   nil,
+   197,   nil,   167,   nil,   nil,   nil,   nil,   nil,   202,   nil,
+   nil,   nil,   nil,   nil,   176,   213,   233,   nil,   nil,   nil,
+   nil,   nil,   nil,   nil,   nil,   nil,   nil,   nil,   nil,   nil,
+   nil,   nil,   nil,   nil,   nil,   nil,   nil,   nil,   nil,   213,
+   nil,   229,   nil,   nil,   213,   nil,   234 ]
 
 racc_goto_check = [
-    35,    35,    35,    27,    40,    42,    26,     8,    32,    12,
-    31,    30,    21,    21,    21,     1,     7,    39,    16,    16,
-    16,     7,    45,    27,    45,    27,    27,    27,    27,    15,
-     5,     6,    15,     6,     9,    30,    15,    15,    15,    37,
-    34,    26,    35,    26,    42,    31,    42,    37,    39,    49,
-    10,     8,    32,    12,    21,    16,    31,    45,    27,    45,
-    27,    32,    38,    32,    11,    13,    34,    15,    38,    14,
-    27,    37,    27,    22,    32,    49,    23,    40,    28,    40,
-    26,    29,    33,    42,    38,    40,    40,    24,    24,    24,
-    24,    41,    43,    35,    44,    47,    48,    27,    50,    35,
-    51,    52,   nil,    40,   nil,    27,   nil,    27,   nil,   nil,
-    35,   nil,   nil,   nil,   nil,   nil,   nil,    35,   nil,    35,
-    40,   nil,   nil,   nil,   nil,    35,   nil,   nil,   nil,   nil,
-    35,    35,   nil,   nil,   nil,   nil,   nil,   nil,   nil,   nil,
-   nil,   nil,   nil,   nil,   nil,   nil,    35,    35 ]
+    37,    37,    37,    27,    26,    42,    44,    32,    39,     8,
+     1,    31,    12,    16,    16,    16,    41,    39,     5,    35,
+    21,    21,    21,    27,     9,    27,    27,    27,    27,    15,
+    10,    47,    15,    47,    36,    51,    15,    15,    15,    26,
+    11,    26,    37,    39,     6,    44,     6,    44,    35,    31,
+    16,    41,    52,     8,    39,    32,    12,    30,    27,    52,
+    27,    31,    21,    36,    51,    13,    47,    15,    47,     7,
+    27,    53,    27,    32,     7,    32,    53,    14,    26,    22,
+    42,    30,    42,    52,    44,    23,    28,    29,    52,    42,
+    42,    33,    32,    37,    40,    43,    45,    27,    46,    37,
+    24,    24,    24,    24,    49,    27,    50,    27,    42,    55,
+    56,    57,    37,   nil,   nil,   nil,   nil,   nil,   nil,   nil,
+    37,   nil,    37,   nil,   nil,   nil,   nil,   nil,    37,   nil,
+   nil,   nil,   nil,   nil,    37,    37,    42,   nil,   nil,   nil,
+   nil,   nil,   nil,   nil,   nil,   nil,   nil,   nil,   nil,   nil,
+   nil,   nil,   nil,   nil,   nil,   nil,   nil,   nil,   nil,    37,
+   nil,    37,   nil,   nil,    37,   nil,    37 ]
 
 racc_goto_pointer = [
-   nil,    15,   nil,   nil,   nil,   -59,    31,    13,   -37,   -55,
-    46,   -23,   -35,    53,    23,    12,     5,   nil,   nil,   nil,
-   nil,   -46,    41,    43,    53,   nil,   -26,    -9,   -32,    11,
-  -107,  -131,  -140,   -82,  -105,   -58,   nil,  -126,  -121,  -140,
-   -88,   -19,   -28,    40,   -38,   -57,   nil,     5,   -49,   -96,
-   -71,   -98,   -24 ]
+   nil,    10,   nil,   nil,   nil,   -71,    44,    66,   -35,   -65,
+    26,   -47,   -32,    53,    31,    12,     0,   nil,   nil,   nil,
+   nil,   -38,    47,    52,    66,   nil,   -28,    -9,   -24,    17,
+   -61,  -130,  -141,   -73,   nil,  -132,  -111,   -58,   nil,  -159,
+   -92,  -141,   -87,   -15,   -27,    44,   -34,   -48,   nil,    14,
+   -39,  -110,  -134,  -148,   nil,   -63,   -99,   -14 ]
 
 racc_goto_default = [
    nil,   nil,     2,     8,    88,   nil,   nil,   nil,   nil,   nil,
    nil,   nil,    10,   nil,   nil,    50,   nil,    21,    22,    23,
    100,   nil,   nil,   nil,   nil,    67,   nil,    75,   nil,   nil,
-   nil,   nil,   nil,   151,   152,    72,   129,   nil,   nil,   167,
-   nil,    76,   nil,   nil,   nil,    80,    90,   nil,   nil,   nil,
-   nil,   nil,   nil ]
+   nil,   nil,   nil,   151,   165,   nil,   152,    72,   129,   nil,
+   nil,   169,   nil,    76,   nil,   nil,   nil,    80,    90,   nil,
+   nil,   nil,   nil,   204,   205,   nil,   nil,   nil ]
 
 racc_reduce_table = [
   0, 0, :racc_error,
-  0, 61, :_reduce_1,
-  2, 61, :_reduce_2,
-  0, 62, :_reduce_3,
-  2, 62, :_reduce_4,
-  1, 63, :_reduce_5,
-  2, 63, :_reduce_6,
-  0, 64, :_reduce_none,
-  1, 64, :_reduce_none,
-  5, 56, :_reduce_none,
-  0, 65, :_reduce_10,
-  0, 66, :_reduce_11,
-  5, 57, :_reduce_12,
-  2, 57, :_reduce_none,
-  1, 71, :_reduce_14,
-  2, 71, :_reduce_15,
-  1, 58, :_reduce_none,
-  2, 58, :_reduce_17,
-  3, 58, :_reduce_none,
-  2, 58, :_reduce_none,
-  2, 58, :_reduce_20,
-  2, 58, :_reduce_21,
-  3, 58, :_reduce_22,
-  2, 58, :_reduce_23,
-  1, 58, :_reduce_24,
-  1, 58, :_reduce_25,
-  2, 58, :_reduce_none,
-  1, 76, :_reduce_27,
-  2, 76, :_reduce_28,
-  2, 67, :_reduce_29,
-  1, 67, :_reduce_none,
-  1, 67, :_reduce_none,
+  0, 64, :_reduce_1,
+  2, 64, :_reduce_2,
+  0, 65, :_reduce_3,
+  2, 65, :_reduce_4,
+  1, 66, :_reduce_5,
+  2, 66, :_reduce_6,
+  0, 67, :_reduce_none,
   1, 67, :_reduce_none,
-  3, 67, :_reduce_33,
-  3, 67, :_reduce_34,
-  3, 67, :_reduce_35,
-  2, 67, :_reduce_36,
-  2, 67, :_reduce_37,
-  2, 67, :_reduce_38,
-  2, 67, :_reduce_39,
-  2, 67, :_reduce_40,
-  2, 72, :_reduce_none,
-  2, 72, :_reduce_42,
-  2, 72, :_reduce_43,
-  2, 72, :_reduce_44,
-  2, 72, :_reduce_45,
-  2, 72, :_reduce_46,
-  1, 81, :_reduce_47,
-  2, 81, :_reduce_48,
-  1, 77, :_reduce_49,
-  2, 77, :_reduce_50,
-  3, 77, :_reduce_51,
-  0, 84, :_reduce_none,
-  1, 84, :_reduce_none,
-  3, 80, :_reduce_54,
+  5, 59, :_reduce_none,
+  0, 68, :_reduce_10,
+  0, 69, :_reduce_11,
+  5, 60, :_reduce_12,
+  2, 60, :_reduce_none,
+  1, 74, :_reduce_14,
+  2, 74, :_reduce_15,
+  1, 61, :_reduce_none,
+  2, 61, :_reduce_17,
+  3, 61, :_reduce_none,
+  2, 61, :_reduce_none,
+  2, 61, :_reduce_20,
+  2, 61, :_reduce_21,
+  3, 61, :_reduce_22,
+  2, 61, :_reduce_23,
+  1, 61, :_reduce_24,
+  1, 61, :_reduce_25,
+  2, 61, :_reduce_none,
+  1, 79, :_reduce_27,
+  2, 79, :_reduce_28,
+  2, 70, :_reduce_29,
+  1, 70, :_reduce_none,
+  1, 70, :_reduce_none,
+  1, 70, :_reduce_none,
+  3, 70, :_reduce_33,
+  3, 70, :_reduce_34,
+  3, 70, :_reduce_35,
+  2, 70, :_reduce_36,
+  2, 70, :_reduce_37,
+  2, 70, :_reduce_38,
+  2, 70, :_reduce_39,
+  2, 70, :_reduce_40,
+  2, 75, :_reduce_none,
+  2, 75, :_reduce_42,
+  2, 75, :_reduce_43,
+  2, 75, :_reduce_44,
+  2, 75, :_reduce_45,
+  2, 75, :_reduce_46,
+  1, 84, :_reduce_47,
+  2, 84, :_reduce_48,
+  1, 80, :_reduce_49,
+  2, 80, :_reduce_50,
+  3, 80, :_reduce_51,
   0, 87, :_reduce_none,
   1, 87, :_reduce_none,
-  8, 73, :_reduce_57,
-  5, 74, :_reduce_58,
-  8, 74, :_reduce_59,
-  1, 85, :_reduce_60,
-  3, 85, :_reduce_61,
-  1, 86, :_reduce_62,
-  3, 86, :_reduce_63,
-  0, 95, :_reduce_none,
-  1, 95, :_reduce_none,
-  1, 88, :_reduce_66,
-  3, 88, :_reduce_67,
-  3, 88, :_reduce_68,
-  6, 88, :_reduce_69,
-  3, 88, :_reduce_70,
-  3, 88, :_reduce_71,
-  0, 83, :_reduce_none,
-  1, 83, :_reduce_73,
-  1, 97, :_reduce_74,
-  2, 97, :_reduce_75,
-  1, 78, :_reduce_76,
-  2, 78, :_reduce_77,
-  3, 78, :_reduce_78,
-  1, 90, :_reduce_none,
+  3, 83, :_reduce_54,
+  0, 90, :_reduce_none,
   1, 90, :_reduce_none,
-  0, 98, :_reduce_81,
-  0, 99, :_reduce_82,
-  5, 70, :_reduce_83,
-  1, 100, :_reduce_84,
-  2, 100, :_reduce_85,
-  1, 79, :_reduce_86,
-  2, 79, :_reduce_87,
-  3, 79, :_reduce_88,
+  8, 76, :_reduce_57,
+  5, 77, :_reduce_58,
+  8, 77, :_reduce_59,
+  1, 88, :_reduce_60,
+  3, 88, :_reduce_61,
+  0, 93, :_reduce_none,
+  1, 93, :_reduce_none,
+  2, 89, :_reduce_64,
+  4, 89, :_reduce_65,
+  0, 100, :_reduce_none,
+  1, 100, :_reduce_none,
+  1, 91, :_reduce_68,
+  3, 91, :_reduce_69,
+  3, 91, :_reduce_70,
+  6, 91, :_reduce_71,
+  3, 91, :_reduce_72,
+  3, 91, :_reduce_73,
+  4, 92, :_reduce_74,
+  0, 86, :_reduce_none,
+  1, 86, :_reduce_76,
+  1, 102, :_reduce_77,
+  2, 102, :_reduce_78,
+  1, 81, :_reduce_79,
+  2, 81, :_reduce_80,
+  3, 81, :_reduce_81,
+  1, 95, :_reduce_none,
+  1, 95, :_reduce_none,
+  0, 103, :_reduce_84,
+  0, 104, :_reduce_85,
+  5, 73, :_reduce_86,
+  1, 105, :_reduce_87,
+  2, 105, :_reduce_88,
   1, 82, :_reduce_89,
-  1, 82, :_reduce_90,
-  0, 102, :_reduce_none,
-  1, 102, :_reduce_none,
-  2, 59, :_reduce_none,
-  2, 59, :_reduce_none,
-  4, 101, :_reduce_95,
-  1, 103, :_reduce_96,
-  3, 103, :_reduce_97,
-  1, 104, :_reduce_98,
-  3, 104, :_reduce_99,
-  5, 104, :_reduce_100,
-  7, 104, :_reduce_101,
-  4, 104, :_reduce_102,
-  3, 104, :_reduce_103,
-  1, 92, :_reduce_104,
-  1, 92, :_reduce_105,
-  1, 92, :_reduce_106,
-  1, 93, :_reduce_107,
-  3, 93, :_reduce_108,
-  2, 93, :_reduce_109,
-  4, 93, :_reduce_110,
-  0, 105, :_reduce_111,
-  0, 106, :_reduce_112,
-  5, 94, :_reduce_113,
-  3, 91, :_reduce_114,
-  0, 107, :_reduce_115,
-  3, 60, :_reduce_116,
-  1, 68, :_reduce_none,
-  0, 69, :_reduce_none,
-  1, 69, :_reduce_none,
-  1, 69, :_reduce_none,
-  1, 69, :_reduce_none,
-  1, 75, :_reduce_none,
-  1, 75, :_reduce_none,
-  0, 89, :_reduce_none,
-  1, 89, :_reduce_none,
-  1, 96, :_reduce_126 ]
-
-racc_reduce_n = 127
-
-racc_shift_n = 217
+  2, 82, :_reduce_90,
+  3, 82, :_reduce_91,
+  1, 85, :_reduce_92,
+  1, 85, :_reduce_93,
+  0, 107, :_reduce_none,
+  1, 107, :_reduce_none,
+  2, 62, :_reduce_none,
+  2, 62, :_reduce_none,
+  4, 106, :_reduce_98,
+  1, 108, :_reduce_99,
+  3, 108, :_reduce_100,
+  1, 109, :_reduce_101,
+  3, 109, :_reduce_102,
+  5, 109, :_reduce_103,
+  7, 109, :_reduce_104,
+  4, 109, :_reduce_105,
+  3, 109, :_reduce_106,
+  1, 97, :_reduce_107,
+  1, 97, :_reduce_108,
+  1, 97, :_reduce_109,
+  1, 98, :_reduce_110,
+  3, 98, :_reduce_111,
+  2, 98, :_reduce_112,
+  4, 98, :_reduce_113,
+  1, 110, :_reduce_114,
+  3, 110, :_reduce_115,
+  2, 110, :_reduce_116,
+  4, 110, :_reduce_117,
+  1, 111, :_reduce_none,
+  1, 111, :_reduce_none,
+  1, 112, :_reduce_120,
+  1, 112, :_reduce_121,
+  0, 113, :_reduce_122,
+  0, 114, :_reduce_123,
+  5, 99, :_reduce_124,
+  3, 96, :_reduce_125,
+  0, 115, :_reduce_126,
+  3, 63, :_reduce_127,
+  1, 71, :_reduce_none,
+  0, 72, :_reduce_none,
+  1, 72, :_reduce_none,
+  1, 72, :_reduce_none,
+  1, 72, :_reduce_none,
+  1, 78, :_reduce_none,
+  1, 78, :_reduce_none,
+  0, 94, :_reduce_none,
+  1, 94, :_reduce_none,
+  1, 101, :_reduce_137 ]
+
+racc_reduce_n = 138
+
+racc_shift_n = 238
 
 racc_token_table = {
   false => 0,
@@ -1067,17 +1092,20 @@ def raise_parse_error(error_message, location)
   "," => 43,
   "|" => 44,
   "%prec" => 45,
-  "{" => 46,
-  "}" => 47,
-  "?" => 48,
-  "+" => 49,
-  "*" => 50,
-  "[" => 51,
-  "]" => 52,
-  "{...}" => 53,
-  "%empty" => 54 }
-
-racc_nt_base = 55
+  "%if" => 46,
+  "{" => 47,
+  "}" => 48,
+  "?" => 49,
+  "+" => 50,
+  "*" => 51,
+  "%true" => 52,
+  "%false" => 53,
+  "[" => 54,
+  "]" => 55,
+  "{...}" => 56,
+  "%empty" => 57 }
+
+racc_nt_base = 58
 
 racc_use_result_var = true
 
@@ -1145,11 +1173,14 @@ def raise_parse_error(error_message, location)
   "\",\"",
   "\"|\"",
   "\"%prec\"",
+  "\"%if\"",
   "\"{\"",
   "\"}\"",
   "\"?\"",
   "\"+\"",
   "\"*\"",
+  "\"%true\"",
+  "\"%false\"",
   "\"[\"",
   "\"]\"",
   "\"{...}\"",
@@ -1188,11 +1219,13 @@ def raise_parse_error(error_message, location)
   "rule_rhs_list",
   "\"-option@TAG\"",
   "rule_rhs",
+  "if_clause",
+  "\"-option@if_clause\"",
   "empty",
   "symbol",
   "named_ref",
   "parameterizing_suffix",
-  "parameterizing_args",
+  "parameterizing_rule_args",
   "midrule_action",
   "\"-option@named_ref\"",
   "string_as_id",
@@ -1204,6 +1237,9 @@ def raise_parse_error(error_message, location)
   "\"-option@;\"",
   "rhs_list",
   "rhs",
+  "parameterizing_args",
+  "symbol_or_bool",
+  "bool",
   "@5",
   "@6",
   "@7" ]
@@ -1646,30 +1682,36 @@ def _reduce_61(val, _values, result)
   end
 .,.,
 
+# reduce 62 omitted
+
+# reduce 63 omitted
+
 module_eval(<<'.,.,', 'parser.y', 201)
-  def _reduce_62(val, _values, result)
+  def _reduce_64(val, _values, result)
                       builder = val[0]
+                  builder.if_clause = val[1]
                   result = [builder]
 
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 206)
-  def _reduce_63(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 207)
+  def _reduce_65(val, _values, result)
                       builder = val[2]
+                  builder.if_clause = val[3]
                   result = val[0].append(builder)
 
     result
   end
 .,.,
 
-# reduce 64 omitted
+# reduce 66 omitted
 
-# reduce 65 omitted
+# reduce 67 omitted
 
-module_eval(<<'.,.,', 'parser.y', 212)
-  def _reduce_66(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 214)
+  def _reduce_68(val, _values, result)
                   reset_precs
               result = Grammar::ParameterizingRule::Rhs.new
 
@@ -1677,8 +1719,8 @@ def _reduce_66(val, _values, result)
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 217)
-  def _reduce_67(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 219)
+  def _reduce_69(val, _values, result)
                   token = val[1]
               token.alias_name = val[2]
               builder = val[0]
@@ -1689,8 +1731,8 @@ def _reduce_67(val, _values, result)
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 225)
-  def _reduce_68(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 227)
+  def _reduce_70(val, _values, result)
                     builder = val[0]
                 builder.symbols << Lrama::Lexer::Token::InstantiateRule.new(s_value: val[2], location: @lexer.location, args: [val[1]])
                 result = builder
@@ -1699,8 +1741,8 @@ def _reduce_68(val, _values, result)
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 231)
-  def _reduce_69(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 233)
+  def _reduce_71(val, _values, result)
                     builder = val[0]
                 builder.symbols << Lrama::Lexer::Token::InstantiateRule.new(s_value: val[1].s_value, location: @lexer.location, args: val[3], lhs_tag: val[5])
                 result = builder
@@ -1709,8 +1751,8 @@ def _reduce_69(val, _values, result)
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 237)
-  def _reduce_70(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 239)
+  def _reduce_72(val, _values, result)
                   user_code = val[1]
               user_code.alias_name = val[2]
               builder = val[0]
@@ -1721,8 +1763,8 @@ def _reduce_70(val, _values, result)
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 245)
-  def _reduce_71(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 247)
+  def _reduce_73(val, _values, result)
                   sym = @grammar.find_symbol_by_id!(val[2])
               @prec_seen = true
               builder = val[0]
@@ -1733,137 +1775,145 @@ def _reduce_71(val, _values, result)
   end
 .,.,
 
-# reduce 72 omitted
+module_eval(<<'.,.,', 'parser.y', 256)
+  def _reduce_74(val, _values, result)
+                  result = Lrama::Lexer::Token::ControlSyntax.new(s_value: val[0], location: @lexer.location, condition: val[2])
 
-module_eval(<<'.,.,', 'parser.y', 253)
-  def _reduce_73(val, _values, result)
-     result = val[0].s_value
     result
   end
 .,.,
 
+# reduce 75 omitted
+
 module_eval(<<'.,.,', 'parser.y', 260)
-  def _reduce_74(val, _values, result)
+  def _reduce_76(val, _values, result)
+     result = val[0].s_value
+    result
+  end
+.,.,
+
+module_eval(<<'.,.,', 'parser.y', 267)
+  def _reduce_77(val, _values, result)
     result = val[1] ? val[1].unshift(val[0]) : val
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 260)
-  def _reduce_75(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 267)
+  def _reduce_78(val, _values, result)
     result = val[1] ? val[1].unshift(val[0]) : val
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 255)
-  def _reduce_76(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 262)
+  def _reduce_79(val, _values, result)
      result = [{tag: nil, tokens: val[0]}]
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 256)
-  def _reduce_77(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 263)
+  def _reduce_80(val, _values, result)
      result = [{tag: val[0], tokens: val[1]}]
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 257)
-  def _reduce_78(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 264)
+  def _reduce_81(val, _values, result)
      result = val[0].append({tag: val[1], tokens: val[2]})
     result
   end
 .,.,
 
-# reduce 79 omitted
+# reduce 82 omitted
 
-# reduce 80 omitted
+# reduce 83 omitted
 
-module_eval(<<'.,.,', 'parser.y', 263)
-  def _reduce_81(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 270)
+  def _reduce_84(val, _values, result)
                    begin_c_declaration("}")
 
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 267)
-  def _reduce_82(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 274)
+  def _reduce_85(val, _values, result)
                    end_c_declaration
 
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 271)
-  def _reduce_83(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 278)
+  def _reduce_86(val, _values, result)
                    result = val[2]
 
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 279)
-  def _reduce_84(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 286)
+  def _reduce_87(val, _values, result)
     result = val[1] ? val[1].unshift(val[0]) : val
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 279)
-  def _reduce_85(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 286)
+  def _reduce_88(val, _values, result)
     result = val[1] ? val[1].unshift(val[0]) : val
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 274)
-  def _reduce_86(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 281)
+  def _reduce_89(val, _values, result)
      result = [{tag: nil, tokens: val[0]}]
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 275)
-  def _reduce_87(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 282)
+  def _reduce_90(val, _values, result)
      result = [{tag: val[0], tokens: val[1]}]
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 276)
-  def _reduce_88(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 283)
+  def _reduce_91(val, _values, result)
      result = val[0].append({tag: val[1], tokens: val[2]})
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 278)
-  def _reduce_89(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 285)
+  def _reduce_92(val, _values, result)
      on_action_error("ident after %prec", val[0]) if @prec_seen
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 279)
-  def _reduce_90(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 286)
+  def _reduce_93(val, _values, result)
      on_action_error("char after %prec", val[0]) if @prec_seen
     result
   end
 .,.,
 
-# reduce 91 omitted
+# reduce 94 omitted
 
-# reduce 92 omitted
+# reduce 95 omitted
 
-# reduce 93 omitted
+# reduce 96 omitted
 
-# reduce 94 omitted
+# reduce 97 omitted
 
-module_eval(<<'.,.,', 'parser.y', 287)
-  def _reduce_95(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 294)
+  def _reduce_98(val, _values, result)
                  lhs = val[0]
              lhs.alias_name = val[1]
              val[3].each do |builder|
@@ -1876,8 +1926,8 @@ def _reduce_95(val, _values, result)
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 298)
-  def _reduce_96(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 305)
+  def _reduce_99(val, _values, result)
                     builder = val[0]
                 if !builder.line
                   builder.line = @lexer.line - 1
@@ -1888,8 +1938,8 @@ def _reduce_96(val, _values, result)
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 306)
-  def _reduce_97(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 313)
+  def _reduce_100(val, _values, result)
                     builder = val[2]
                 if !builder.line
                   builder.line = @lexer.line - 1
@@ -1900,8 +1950,8 @@ def _reduce_97(val, _values, result)
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 315)
-  def _reduce_98(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 322)
+  def _reduce_101(val, _values, result)
                reset_precs
            result = @grammar.create_rule_builder(@rule_counter, @midrule_action_counter)
 
@@ -1909,8 +1959,8 @@ def _reduce_98(val, _values, result)
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 320)
-  def _reduce_99(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 327)
+  def _reduce_102(val, _values, result)
                token = val[1]
            token.alias_name = val[2]
            builder = val[0]
@@ -1921,8 +1971,8 @@ def _reduce_99(val, _values, result)
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 328)
-  def _reduce_100(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 335)
+  def _reduce_103(val, _values, result)
                token = Lrama::Lexer::Token::InstantiateRule.new(s_value: val[2], alias_name: val[3], location: @lexer.location, args: [val[1]], lhs_tag: val[4])
            builder = val[0]
            builder.add_rhs(token)
@@ -1933,8 +1983,8 @@ def _reduce_100(val, _values, result)
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 336)
-  def _reduce_101(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 343)
+  def _reduce_104(val, _values, result)
                token = Lrama::Lexer::Token::InstantiateRule.new(s_value: val[1].s_value, alias_name: val[5], location: @lexer.location, args: val[3], lhs_tag: val[6])
            builder = val[0]
            builder.add_rhs(token)
@@ -1945,8 +1995,8 @@ def _reduce_101(val, _values, result)
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 344)
-  def _reduce_102(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 351)
+  def _reduce_105(val, _values, result)
                user_code = val[1]
            user_code.alias_name = val[2]
            user_code.tag = val[3]
@@ -1958,8 +2008,8 @@ def _reduce_102(val, _values, result)
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 353)
-  def _reduce_103(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 360)
+  def _reduce_106(val, _values, result)
                sym = @grammar.find_symbol_by_id!(val[2])
            @prec_seen = true
            builder = val[0]
@@ -1970,57 +2020,103 @@ def _reduce_103(val, _values, result)
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 360)
-  def _reduce_104(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 367)
+  def _reduce_107(val, _values, result)
      result = "option"
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 361)
-  def _reduce_105(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 368)
+  def _reduce_108(val, _values, result)
      result = "nonempty_list"
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 362)
-  def _reduce_106(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 369)
+  def _reduce_109(val, _values, result)
      result = "list"
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 364)
-  def _reduce_107(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 371)
+  def _reduce_110(val, _values, result)
      result = [val[0]]
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 365)
-  def _reduce_108(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 372)
+  def _reduce_111(val, _values, result)
      result = val[0].append(val[2])
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 366)
-  def _reduce_109(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 373)
+  def _reduce_112(val, _values, result)
      result = [Lrama::Lexer::Token::InstantiateRule.new(s_value: val[1].s_value, location: @lexer.location, args: val[0])]
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 367)
-  def _reduce_110(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 374)
+  def _reduce_113(val, _values, result)
      result = [Lrama::Lexer::Token::InstantiateRule.new(s_value: val[0].s_value, location: @lexer.location, args: val[2])]
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 371)
-  def _reduce_111(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 376)
+  def _reduce_114(val, _values, result)
+     result = [val[0]]
+    result
+  end
+.,.,
+
+module_eval(<<'.,.,', 'parser.y', 377)
+  def _reduce_115(val, _values, result)
+     result = val[0].append(val[2])
+    result
+  end
+.,.,
+
+module_eval(<<'.,.,', 'parser.y', 378)
+  def _reduce_116(val, _values, result)
+     result = [Lrama::Lexer::Token::InstantiateRule.new(s_value: val[1].s_value, location: @lexer.location, args: val[0])]
+    result
+  end
+.,.,
+
+module_eval(<<'.,.,', 'parser.y', 379)
+  def _reduce_117(val, _values, result)
+     result = [Lrama::Lexer::Token::InstantiateRule.new(s_value: val[0].s_value, location: @lexer.location, args: val[2])]
+    result
+  end
+.,.,
+
+# reduce 118 omitted
+
+# reduce 119 omitted
+
+module_eval(<<'.,.,', 'parser.y', 384)
+  def _reduce_120(val, _values, result)
+     result = Lrama::Lexer::Token::Ident.new(s_value: true)
+    result
+  end
+.,.,
+
+module_eval(<<'.,.,', 'parser.y', 385)
+  def _reduce_121(val, _values, result)
+     result = Lrama::Lexer::Token::Ident.new(s_value: false)
+    result
+  end
+.,.,
+
+module_eval(<<'.,.,', 'parser.y', 389)
+  def _reduce_122(val, _values, result)
                           if @prec_seen
                         on_action_error("multiple User_code after %prec", val[0])  if @code_after_prec
                         @code_after_prec = true
@@ -2031,31 +2127,31 @@ def _reduce_111(val, _values, result)
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 379)
-  def _reduce_112(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 397)
+  def _reduce_123(val, _values, result)
                           end_c_declaration
 
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 383)
-  def _reduce_113(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 401)
+  def _reduce_124(val, _values, result)
                           result = val[2]
 
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 386)
-  def _reduce_114(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 404)
+  def _reduce_125(val, _values, result)
      result = val[1].s_value
     result
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 390)
-  def _reduce_115(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 408)
+  def _reduce_126(val, _values, result)
                                 begin_c_declaration('\Z')
                             @grammar.epilogue_first_lineno = @lexer.line + 1
 
@@ -2063,8 +2159,8 @@ def _reduce_115(val, _values, result)
   end
 .,.,
 
-module_eval(<<'.,.,', 'parser.y', 395)
-  def _reduce_116(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 413)
+  def _reduce_127(val, _values, result)
                                 end_c_declaration
                             @grammar.epilogue = val[2].s_value
 
@@ -2072,26 +2168,26 @@ def _reduce_116(val, _values, result)
   end
 .,.,
 
-# reduce 117 omitted
+# reduce 128 omitted
 
-# reduce 118 omitted
+# reduce 129 omitted
 
-# reduce 119 omitted
+# reduce 130 omitted
 
-# reduce 120 omitted
+# reduce 131 omitted
 
-# reduce 121 omitted
+# reduce 132 omitted
 
-# reduce 122 omitted
+# reduce 133 omitted
 
-# reduce 123 omitted
+# reduce 134 omitted
 
-# reduce 124 omitted
+# reduce 135 omitted
 
-# reduce 125 omitted
+# reduce 136 omitted
 
-module_eval(<<'.,.,', 'parser.y', 412)
-  def _reduce_126(val, _values, result)
+module_eval(<<'.,.,', 'parser.y', 430)
+  def _reduce_137(val, _values, result)
      result = Lrama::Lexer::Token::Ident.new(s_value: val[0])
     result
   end
diff --git a/parser.y b/parser.y
index 52603a72..75351b9c 100644
--- a/parser.y
+++ b/parser.y
@@ -197,14 +197,16 @@ rule
   rule_args: IDENTIFIER { result = [val[0]] }
            | rule_args "," IDENTIFIER { result = val[0].append(val[2]) }
 
-  rule_rhs_list: rule_rhs
+  rule_rhs_list: rule_rhs if_clause?
                 {
                   builder = val[0]
+                  builder.if_clause = val[1]
                   result = [builder]
                 }
-          | rule_rhs_list "|" rule_rhs
+          | rule_rhs_list "|" rule_rhs if_clause?
                 {
                   builder = val[2]
+                  builder.if_clause = val[3]
                   result = val[0].append(builder)
                 }
 
@@ -227,7 +229,7 @@ rule
                 builder.symbols << Lrama::Lexer::Token::InstantiateRule.new(s_value: val[2], location: @lexer.location, args: [val[1]])
                 result = builder
               }
-          | rule_rhs IDENTIFIER "(" parameterizing_args ")" TAG?
+          | rule_rhs IDENTIFIER "(" parameterizing_rule_args ")" TAG?
               {
                 builder = val[0]
                 builder.symbols << Lrama::Lexer::Token::InstantiateRule.new(s_value: val[1].s_value, location: @lexer.location, args: val[3], lhs_tag: val[5])
@@ -250,6 +252,11 @@ rule
               result = builder
             }
 
+  if_clause: "%if" "(" IDENTIFIER ")"
+            {
+              result = Lrama::Lexer::Token::ControlSyntax.new(s_value: val[0], location: @lexer.location, condition: val[2])
+            }
+
   alias: # empty
        | string_as_id { result = val[0].s_value }
 
@@ -362,11 +369,22 @@ rule
                        | "+" { result = "nonempty_list" }
                        | "*" { result = "list" }
 
-  parameterizing_args: symbol { result = [val[0]] }
-                     | parameterizing_args ',' symbol { result = val[0].append(val[2]) }
+  parameterizing_rule_args: symbol { result = [val[0]] }
+                          | parameterizing_args ',' symbol { result = val[0].append(val[2]) }
+                          | symbol parameterizing_suffix { result = [Lrama::Lexer::Token::InstantiateRule.new(s_value: val[1].s_value, location: @lexer.location, args: val[0])] }
+                          | IDENTIFIER "(" parameterizing_args ")" { result = [Lrama::Lexer::Token::InstantiateRule.new(s_value: val[0].s_value, location: @lexer.location, args: val[2])] }
+
+  parameterizing_args: symbol_or_bool { result = [val[0]] }
+                     | parameterizing_args ',' symbol_or_bool { result = val[0].append(val[2]) }
                      | symbol parameterizing_suffix { result = [Lrama::Lexer::Token::InstantiateRule.new(s_value: val[1].s_value, location: @lexer.location, args: val[0])] }
                      | IDENTIFIER "(" parameterizing_args ")" { result = [Lrama::Lexer::Token::InstantiateRule.new(s_value: val[0].s_value, location: @lexer.location, args: val[2])] }
 
+  symbol_or_bool: symbol
+                | bool
+
+  bool: "%true" { result = Lrama::Lexer::Token::Ident.new(s_value: true) }
+      | "%false" { result = Lrama::Lexer::Token::Ident.new(s_value: false) }
+
   midrule_action: "{"
                     {
                       if @prec_seen
diff --git a/sig/lrama/grammar/parameterizing_rule/rhs.rbs b/sig/lrama/grammar/parameterizing_rule/rhs.rbs
index 844557c9..bc207d52 100644
--- a/sig/lrama/grammar/parameterizing_rule/rhs.rbs
+++ b/sig/lrama/grammar/parameterizing_rule/rhs.rbs
@@ -7,6 +7,7 @@ module Lrama
         attr_reader precedence_sym: Grammar::Symbol?
 
         def initialize: () -> void
+        def skip?: (Grammar::Binding bindings) -> bool
         def resolve_user_code: (Grammar::Binding bindings) -> Lexer::Token::UserCode?
       end
     end
diff --git a/sig/lrama/lexer/token/control_syntax.rbs b/sig/lrama/lexer/token/control_syntax.rbs
new file mode 100644
index 00000000..c6f06114
--- /dev/null
+++ b/sig/lrama/lexer/token/control_syntax.rbs
@@ -0,0 +1,15 @@
+module Lrama
+  class Lexer
+    class Token
+      class ControlSyntax < Token
+        attr_accessor condition: Lexer::Token::Ident?
+
+        def initialize: (s_value: String, location: Location, ?condition: Lexer::Token::Ident?) -> void
+        def if?: () -> bool
+        def true?: () -> bool
+        def false?: () -> bool
+        def condition_value: () -> String?
+      end
+    end
+  end
+end
diff --git a/spec/fixtures/parameterizing_rules/user_defined/if.y b/spec/fixtures/parameterizing_rules/user_defined/if.y
new file mode 100644
index 00000000..ffacc836
--- /dev/null
+++ b/spec/fixtures/parameterizing_rules/user_defined/if.y
@@ -0,0 +1,43 @@
+/*
+ * This is comment for this file.
+ */
+
+%{
+// Prologue
+static int yylex(YYSTYPE *val, YYLTYPE *loc);
+static int yyerror(YYLTYPE *loc, const char *str);
+%}
+
+%union {
+    int i;
+}
+
+%token <i> number
+
+%rule defined_rule(X, condition): /* empty */
+                                | X { $$ = $1; } %if(condition)
+                                ;
+
+%%
+
+r_true        : defined_rule(number, %true)
+              ;
+
+r_false       : defined_rule(number, %false)
+              ;
+
+%%
+
+static int yylex(YYSTYPE *yylval, YYLTYPE *loc)
+{
+  return 0;
+}
+
+static int yyerror(YYLTYPE *loc, const char *str)
+{
+  return 0;
+}
+
+int main(int argc, char *argv[])
+{
+}
diff --git a/spec/lrama/parser_spec.rb b/spec/lrama/parser_spec.rb
index 8b14ee5f..3eb2110d 100644
--- a/spec/lrama/parser_spec.rb
+++ b/spec/lrama/parser_spec.rb
@@ -2334,6 +2334,86 @@
             ])
           end
         end
+
+        context "when if" do
+          let(:path) { "parameterizing_rules/user_defined/if.y" }
+
+          it "expands parameterizing rules" do
+            expect(grammar.nterms.sort_by(&:number)).to match_symbols([
+              Sym.new(id: T::Ident.new(s_value: "$accept"), alias_name: nil, number: 4, tag: nil, term: false, token_id: 0, nullable: false),
+              Sym.new(id: T::Ident.new(s_value: "defined_rule_number_true"), alias_name: nil, number: 5, tag: nil, term: false, token_id: 1, nullable: true),
+              Sym.new(id: T::Ident.new(s_value: "r_true"), alias_name: nil, number: 6, tag: nil, term: false, token_id: 2, nullable: true),
+              Sym.new(id: T::Ident.new(s_value: "defined_rule_number_false"), alias_name: nil, number: 7, tag: nil, term: false, token_id: 3, nullable: true),
+              Sym.new(id: T::Ident.new(s_value: "r_false"), alias_name: nil, number: 8, tag: nil, term: false, token_id: 4, nullable: true),
+            ])
+
+            expect(grammar.rules).to eq([
+              Rule.new(
+                id: 0,
+                lhs: grammar.find_symbol_by_s_value!("$accept"),
+                rhs: [
+                  grammar.find_symbol_by_s_value!("r_true"),
+                  grammar.find_symbol_by_s_value!("YYEOF"),
+                ],
+                token_code: nil,
+                nullable: false,
+                precedence_sym: grammar.find_symbol_by_s_value!("YYEOF"),
+                lineno: 23,
+              ),
+              Rule.new(
+                id: 1,
+                lhs: grammar.find_symbol_by_s_value!("defined_rule_number_true"),
+                rhs: [],
+                token_code: nil,
+                nullable: true,
+                precedence_sym: nil,
+                lineno: 23,
+              ),
+              Rule.new(
+                id: 2,
+                lhs: grammar.find_symbol_by_s_value!("defined_rule_number_true"),
+                rhs: [
+                  grammar.find_symbol_by_s_value!("number"),
+                ],
+                token_code: T::UserCode.new(s_value: " $$ = $1; "),
+                nullable: false,
+                precedence_sym: grammar.find_symbol_by_s_value!("number"),
+                lineno: 23,
+              ),
+              Rule.new(
+                id: 3,
+                lhs: grammar.find_symbol_by_s_value!("r_true"),
+                rhs: [
+                  grammar.find_symbol_by_s_value!("defined_rule_number_true"),
+                ],
+                token_code: nil,
+                nullable: true,
+                precedence_sym: nil,
+                lineno: 23,
+              ),
+              Rule.new(
+                id: 4,
+                lhs: grammar.find_symbol_by_s_value!("defined_rule_number_false"),
+                rhs: [],
+                token_code: nil,
+                nullable: true,
+                precedence_sym: nil,
+                lineno: 26,
+              ),
+              Rule.new(
+                id: 5,
+                lhs: grammar.find_symbol_by_s_value!("r_false"),
+                rhs: [
+                  grammar.find_symbol_by_s_value!("defined_rule_number_false")
+                ],
+                token_code: nil,
+                nullable: true,
+                precedence_sym: nil,
+                lineno: 26,
+              ),
+            ])
+          end
+        end
       end
 
       context 'when error case' do