SICP 問題 2.62
(define (union-set s t) (cond ((null? s) t) ((null? t) s) ((= (car s) (car t)) (cons (car s) (union-set (cdr s) (cdr t)))) ((< (car s) (car t)) (cons (car s) (union-set (cdr s) t))) ((< (car t) (car s)) (cons (car t) (union-set s (cdr t))))))