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;                          DRAW a BINARY TREE
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(define (draw-btree bnode)
  (let ((y-inc 20))
    (define (draw-beetree bnode x-lo x x-hi y)
      (cond ((and (not (empty? bnode)) (not (empty? (left bnode))))
	     (let ((lx (quotient (+ x-lo x -1) 2))
		   (ly (+ y y-inc)))
	       (draw-line x y (quotient (+ x-lo x -1) 2) ly 2)
	       (draw-beetree (left bnode) x-lo lx x ly))))
      (cond ((and (not (empty? bnode)) (not (empty? (right bnode))))
	     (let ((rx (quotient (+ x x-hi 1) 2))
		   (ry (+ y y-inc)))
	       (draw-line x y rx ry 2)
	       (draw-beetree (right bnode) x rx x-hi ry)))))
    (clear-graphics!)
    (draw-beetree bnode 20 300 600 30)))