2.59.rkt

#lang sicp

(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 (union-set set1 set2)
  (cond ((null? set1) set2)
        (else (union-set (cdr set1) (adjoin-set (car set1) set2)))))