gistfile1.scm

(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)))))

set.scm

;; Sets as unordered lists;


;; do not allow duplicates
(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)
	((null? set2) set1)
	((element-of-set? (car set1) set2)
	 (union-set (cdr set1) set2))
	(else 
	 (cons (car set1) set2)
	 (union-set (cdr set1) set2))))

;; allow duplicates

(define (adjoin-set x set)
  (cons x set))

two versions of union-set, one is iterative process, the other is recursive

Duplicate version is preferable when the problem needs more constructing than selecting.