WINDOW

	tells the turtle to enter window mode:  From now on, if the turtle
	is asked to move past the boundary of the graphics window, it
	will move offscreen.  The visible graphics window is considered
	as just part of an infinite graphics plane; the turtle can be
	anywhere on the plane.  (If you lose the turtle, HOME will bring
	it back to the center of the window.)  Compare WRAP and FENCE.