;; then

(define (element-of-set? x set)
  (cond ((null? set) #f)
	((= x (entry set)) #t)
	((< x (entry set))
	 (element-of-set? x (left-branch set)))
	((> x (entry set))
	 (element-of-set? x (right-branch set)))))


(define (adjoin-set x set)
  (cond ((null? set) (make-tree x '() '()))
	((= x (entry set)) set)
	((< x (entry set))
	 (make-tree (entry set)
		    (adjoint-set x (left-branch set))
		    (right-branch set)))
	((> x (entry set))
	 (make-tree (entry set)
		    (left-branch set)
		    (adjoint-set x (left-branch set))))))