1 (define (cycle? x)
2   (define (compare end-index current-index z current-list)
3     (cond ((eq? end-index current-index) false)
4 	  ((eq? current-list z) true)
5 	  (else (compare end-index
6 			 (+ 1 current-index)
7 			 z
8 			 (cdr current-list)))))
9   (define (find-cycle z i)
10     (cond ((null? z) false)
11 	  ((compare i 0 z x) true)
12 	  (else (find-cycle (cdr z) (+ 1 i)))))
13   (if (not (pair? x))
14       (error "Argument of cycle? must be a pair")
15       (find cycle (cdr x) 1)))