(define (element-of-set? x set)
(cond ((null? set) #f)
((equal? x (car set)) #t)
(else (element-of-set? x (cdr set)))))
(define (adjoin-set x set)
(if (element-of-set? x set) set
(cons x set)))
(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))))
(define (union-set set1 set2)
(cond ((or (null? set1) (null? set2)) (append set1 set2))
((element-of-set? (car set1) set2)
(union-set (cdr set1) set2))
(else (cons (car set1) (union-set (cdr set1) set2)))))
(union-set (list 1 2 3) (list 1 2 3))
(union-set (list 1 2 3) (list 1 2 3 4))
(union-set (list 1 2 3) (list 4 5 6))