연습문제 2.65

(define (union-set set1 set2)
  (let ((list1 (tree->list-1 set1)) (list2 (tree->list-1 set2)))
    (let ((list-union (union-list list1 list2)))
      (let ((union (list->tree list-union)))
        union))))

(define (intersection-set set1 set2)
  (let ((list1 (tree->list-1 set1)) (list2 (tree->list-1 set2)))
    (let ((list-intersection (intersection-list list1 list2)))
      (let ((intersection (list->tree list-intersection)))
        intersection))))