(define (after-2 x)
  (if (null? (cdr x)) '()
      (cddr x)))

(define (cycle? lst)
  (define (visit x y)
    (cond ((or (null? x) (null? y)) #f)
          ((eq? x y) #t)
          (else
           (visit (cdr x) (after-2 y)))))
  (visit lst (after-2 lst)))