throw error on cycles

[jcubic]

Some procedures like:

• reverse
• list->array
• length

break the loop on cycles (they return 2 elements with this code):

(define x (let ((x '(1 2 3))) (set-cdr! (cddr x) x) x))
(length x)
;; ==> 2
(list->array x)
;; ==> #(1 2)
(reverse x)
;; ==> (2 1)

They should throw an exception that the argument is not a list, similar to map:

(map - x)
;; map: argument 1 is not a list