3.17.rkt

(define (count-pairs x)
  (let ((past-list '()))
    (define (pointer-in-list? p l)
      (cond ((null? l) #f)
            ((eq? p (car l)) #t)
            (else (pointer-in-list? p (cdr l)))))
    (define (append-cons l p-list)
      (set! p-list (cons (caar l) p-list))
      p-list)
    (define (iter list past-list n)
      (cond ((not (pair? list)) n)
            ((pointer-in-list? (cdr list) past-list) n)
            (else
             (+ (iter (cdr list) (append-cons list past-list) (+ n 1))
                (iter (car list) past-list 0)))))
    (iter x past-list 0)))                                        

(define z (cons 1 (cons (list 1 2) (list 1))))
(define p
  (let ((x (cons 1 (cons (list 1 2) (list 1)))))
    (set-cdr! (cdadr x) (cadr x))
    x))

(define (pointer-in-list? p l)
    (cond ((null? l) #f)
          ((eq? p (car l)) #t)
          (else (pointer-in-list? p (cdr l)))))