Ex-2.65

;set1, set2 are ordered lists
(define (union-set-aux set1 set2)
  (cond
   ((null? set1) set2)
   ((null? set2) set1)
   ((< (car set1) (car set2))
    (cons (car set1) (union-set-aux (cdr set1) set2)))
   ((= (car set1) (car set2))
    (cons (car set1) (union-set-aux (cdr set1) (cdr set2))))
   (else
    (union-set-aux set2 set1))))
;set1, set2 are binary trees
(define (union-set set1 set2)
  (list->tree
   (union-set-aux
    (tree->list-2 set1)
    (tree->list-2 set2))))