anchor.diff:  (defstruct screen
     name
     framebuffer
  +  backbuffer
  +  flip-function
     relative-position)
   
   (defvar *screens* '())
  
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: (defun compose-all-displays ()
    (when *screens*
      (let* ((screen (first *screens*))
             (fb (screen-backbuffer screen))
             (dims (array-dimensions fb))
             (width (second dims))
             (height (first dims)))
  