(define koch
  (let ((s (/ (sqrt 3) 2 3)))
    (lambda (res depth)
      (letrec
	  ((iter
	    (lambda (x1 y1 x2 y2 d)
	      (if (zero? d) 
		  (draw-line x2 y2)
		  (let* ((dx (- x2 x1)) 
			 (dy (- y2 y1))
			 (thx (+ x1 (/ dx 3))) 
			 (thy (+ y1 (/ dy 3)))
			 (thx2 (+ x1 (* 2 (/ dx 3)))) 
			 (thy2 (+ y1 (* 2 (/ dy 3))))
			 (mx (/ (+ x1 x2) 2)) 
			 (my (/ (+ y1 y2) 2))
			 (midx (+ mx (* (- dy) s))) 
			 (midy (+ my (* dx s))))
		    (iter x1 y1 thx thy (- d 1))
		    (iter thx thy midx midy (- d 1))
		    (iter midx midy thx2 thy2 (- d 1))
		    (iter thx2 thy2 x2 y2 (- d 1)))))))
	(draw-move 0 0)
	(draw-color 0 255 0)
	(iter 0 0 res 0 depth)))))

(koch 200 4)