SICP 問題 2.95
(define (element-of-set? x set) (cond ((null? set) false) ((equal? x (car set)) true) (else (element-of-set? x (cdr set))))) (define (unionset s t) (cond ((null? s) t) ((element-of-set? (car s) t) (unionset (cdr s) t)) (else (cons (car s) (unionset (cdr s) t)))))