(define (cycle? list)
  (define (has-eq? pair list)
    (cond
      [(null? list) #f]
      [(eq? pair list) #t]
      [else (has-eq? pair (cdr list))]))

  (cond
    [(null? list) #f]
    [(has-eq? list (cdr list)) #t]
    [else (cycle? (cdr list))]))