(define (pre-order tree)
  (cond ((null? tree) '())
	(else (print (entry tree))
	      (pre-order (left-branch tree))
	      (pre-order (right-branch tree)) )))

(define (in-order tree)
  (cond ((null? tree) '())
	(else (in-order (left-branch tree))
	      (print (entry tree))
	      (in-order (right-branch tree)) )))

(define (post-order tree)
  (cond ((null? tree) '())
	(else (post-order (left-branch tree))
	      (post-order (right-branch tree))
	      (print (entry tree)) )))