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))