anchor.diff:           (frame (search-previous-frame vf (virtual-frame-current vf))))
       (when frame
         (setf (virtual-frame-current vf) frame)
  -      (lem:map-frame (implementation) frame))
  +      (map-frame (implementation) frame))
       (lem::change-display-size-hook)
       (setf (virtual-frame-changed vf) t)))
   
  
positive.code:          (id (find-unused-frame-id vf)))
      (when (null id)
        (editor-error "it's full of frames in virtual frame"))
      (let ((frame (lem:make-frame (current-frame))))
        (lem:setup-frame frame (primordial-buffer))
  
        (setf (virtual-frame-current vf) frame)
        (allocate-frame vf frame)
        (lem:map-frame (implementation) frame)
  
        (setf (virtual-frame-changed vf) t))))
  
  
negative.code:     (let ((frame (search-previous-frame vf (virtual-frame-current vf))))
        (free-frame vf (virtual-frame-current vf))
        (setf (virtual-frame-current vf) frame)
        (map-frame (implementation) frame))
      (setf (virtual-frame-changed vf) t)))
  
  (define-key *global-keymap* "C-z p" 'frame-multiplexer-prev)
  