Ex-3.17

(define (count-pairs x)
  (define (stored-in-list? x l)
    (cond
     ((null? l) #f)
     ((eq? (car l) x) #t)
     (else (stored-in-list? x (cdr l)))))
  (let ((cache nil))
    (define (count-pairs-aux x)
      (cond
       ((not (pair? x)) 0)
       ((stored-in-list? x cache)
        (+ (count-pairs-aux (car x))
           (count-pairs-aux (cdr x))))
       (else
        (set! cache (cons x cache))
        (+ (count-pairs-aux (car x))
           (count-pairs-aux (cdr x))
           1))))
    (count-pairs-aux x)))