(define (cycle? ls)  

(define (catch? slow fast)

    (cond ((eq? slow fast) #t)

          ((or (null? slow) (null? fast)) #f)

          ((or (null? (cdr slow)) (null? (cdr fast))) #f)

          (else (catch? (cdr slow) (cddr fast)))))

  

  (if (and (pair? ls) (pair? (cdr ls))) (catch? (cdr ls) (cddr ls))

      #f))



네 토끼와 거북이 

리스트 프로세싱 언어합시다