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