anchor.diff:  
   (defun switch-current-frame (virtual-frame frame)
     (setf (virtual-frame-current virtual-frame) frame)
  +  (setf (virtual-frame-changed virtual-frame) t)
     (map-frame (implementation) frame))
   
   (defun find-unused-frame-id (virtual-frame)
  
positive.code:       (editor-error "cannot delete this virtual frame"))
      (let ((frame (search-previous-frame vf (virtual-frame-current vf))))
        (free-frame vf (virtual-frame-current vf))
        (switch-current-frame vf frame))
      (setf (virtual-frame-changed vf) t)))
  
  (define-key *global-keymap* "C-z p" 'frame-multiplexer-prev)
  (define-command frame-multiplexer-prev () ()
  
negative.code:     (let ((frame (make-frame (current-frame))))
        (setup-frame frame (primordial-buffer))
        (allocate-frame vf frame)
        (switch-current-frame vf frame))))
  
  (define-key *global-keymap* "C-z d" 'frame-multiplexer-delete)
  (define-command frame-multiplexer-delete () ()
  