Type the following commands into the scheme interpreter.
> (open-graphics) ; nothing appears to happen
; but behind the scenes ...
> (define w (open-viewport "practice" 300 300)) ; viewport appears
; place viewport with mouse
> ((draw-line w) (make-posn 30 30) (make-posn 100 100)) ; line appears
> (close-viewport w) ; viewport disappears
> (close-graphics) ; again, nothing appears to happen
; unclosed viewports (if any) would disappear