2015/11/19
SICP 問題 3.17
任意の構造の異なる対の個数を返すcount-pairsを完成させる.
(define (count-pairs x)
(define pair-list '())
(define (recur s)
(cond ((not (pair? x)) 0)
((memq s pair-list) 0)
(else
(set! pair-list (cons s pair-list))
(+ (recur (car x))
(recur (cdr x))
1))))
(recur s))