Ex-3.18

(define (has-cycle? list)
  (define (already-in-list? x l)
    (cond
     ((null? l) #f)
     ((eq? (car l) x) #t)
     (else (already-in-list? x (cdr l)))))
  (let ((cache nil))
    (define (has-cycle-aux? list)
      (cond
       ((null? list) #f)
       ((already-in-list? (car list) cache)
        #t)
       (else
        (set! cache (cons (car list) cache))
        (has-cycle-aux? (cdr list)))))
    (has-cycle-aux? list)))