(define true #t)

(define false #f)

(define (element-of-set? x set) 
  (cond ((null? set) false) 
        ((equal? x (car set)) true) 
        (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)
        ((null? set2) set1)
        ((element-of-set? (car set1) set2)
         (union-set (cdr set1) set2))
        (else (cons (car set1) (union-set (cdr set1) set2)))))
