1 (define (cycle? x)
2   (let ((aux '()))
3     (define (find-cycle z)
4       (cond ((null? z) false)
5 	    ((memq z aux) true)
6 	    (else 
7 	     (set-cdr! (last-pair aux) (list z))
8 	     (find-cycle (cdr z)))))
9     (if (not (pair? x))
10 	(error "argument of cycle? must be a pair."))
11     (set! aux (list x))
12     (find-cycle (cdr x))))