anchor.diff:  (define-key *global-keymap* "C-z p" 'fm-prev)
   (define-command fm-prev () ()
     (block exit
  +    (when (null *vf-map*)
  +      ;; ERROR: variable `frame-multiplexer- is broken?
  +      (return-from exit))
       (let* ((vf (gethash (implementation) *vf-map*))
              (id (position (vf-current vf) (vf-frames vf))))
         (when (null id)
  
positive.code: (define-key *global-keymap* "c-z c" 'fm-create)
  (define-command fm-create () ()
    (block exit
      (let* ((vf (gethash (implementation) *vf-map*))
             (id (position-if #'null (vf-frames vf))))
        (when (null id)
  
negative.code: (define-key *global-keymap* "c-z d" 'fm-delete)
  (define-command fm-delete () ()
    (block exit
      (when (null *vf-map*)
        ;; ERROR: variable `frame-multiplexer- is broken?
        (return-from exit))
      (let* ((vf (gethash (implementation) *vf-map*))
             (num (count-if-not #'null (vf-frames vf)))
             (id (position (vf-current vf) (vf-frames vf))))
  