(define (element-of-set? x set)
(cond ((null? set) #f)
((equal? x (car set)) #t)
(else (element-of-set? x (cdr set)))))
(element-of-set? 4 '(1 2 3 2 3 1 5 4))
(element-of-set? 6 '(1 2 3 2 3 1 5 4))
(define adjoin-set cons)
(adjoin-set 4 '(1 2 4))
(define union-set append)
(union-set '(1 2 3 5 9) '(1 2 4))
(define (intersection-set set1 set2)
(cond ((or (null? set1) (null? set2)) '())
((element-of-set? (car set1) set2)
(cons (car set1)
(intersection-set (cdr set1) set2)))
(else (intersection-set (cdr set1) set2))))
(intersection-set '(1 2 5 6 8 4 3) '(1 2 3 4))
(intersection-set '(1 8) '(7 2))