anchor.diff:            (when (window-visiblep window)
               (blit-window fb window)))
           (when (window-visiblep *window-list*)
  -          (blit-window fb *window-list*))))))
  +          (blit-window fb *window-list*)))
  +      (rotatef (screen-backbuffer screen) (screen-framebuffer screen))
  +      (funcall (screen-flip-function screen)))))
   
   (defgeneric key-press-event (window character))
   (defmethod key-press-event ((window window) character))
  
positive.code: (defun compose-all-displays ()
    (when *screens*
      (let* ((screen (first *screens*))
             (fb (screen-framebuffer screen))
             (dims (array-dimensions fb))
             (width (second dims))
             (height (first dims)))
  
negative.code: (defstruct screen
    name
    framebuffer
    backbuffer
    flip-function
    relative-position)
  
  (defvar *screens* '())
  