Protecting Graphics Operations

To guarantee the proper closing of viewports in cases of errors, especially when a program manages several viewports simultaneously, a programmer should use dynamic-wind:

> (let ([w (open-viewport "hello" 100 100)])
    (dynamic-wind
      ; what we want to happen first: nothing
      void
      
      ; the main program (errors constrained to this piece)
      (lambda () (draw-pixel 13))  ; an error

      ; what we would like to happen, whether the main program finishes 
      ; normally or not
      (lambda () (close-viewport w))))