SICP 問題 3.15

(define x (list 'a 'b))
(define z1 (cons x x))
(define z2 (cons (list 'a 'b) (list 'a 'b)))

(define (set-to-wow! x)
  (set-car! (car x) 'wow)
  x)

(set-to-wow! z1)と(set-to-wow! z2)の結果の箱とポインタ図