2.60.scm

(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 ((null? set1) set2)
;; do not check duplicated elements
;        ((element-of-set? (car set1) set2)
;         (union-set (cdr set1) set2))
        (else (cons (car set1)
                    (union-set (cdr set1) set2)))))

(intersection-set '(a e b a c d e) '(f g a e h))
(union-set '(a b c d e) '(f g a e h))