3 (define (union-set set1 set2)
4 (cond ((null? set1) set2)
6 ((= (car set1) (car set2)) (cons (car set1) (union-set (cdr set1) (cdr set2))))
7 ((< (car set1) (car set2)) (cons (car set1) (union-set (cdr set1) set2)))
8 ((> (car set1) (car set2)) (cons (car set2) (union-set set1 (cdr set2))))))