anchor.diff:    (mark (view cursor)))
   
   (defmethod enabled ((cursor mark-cursor))
  -  *show-mark*)
  +  (and (call-next-method) *show-mark*))
   
   (defgeneric visible-1 (cursor view)
     (:documentation "Is `cursor', associated with `view', visible?
  
positive.code:   (if (active cursor)
        (active-ink cursor)
        (inactive-ink cursor)))
  
  (defclass point-cursor (drei-cursor)
    ()
  
negative.code:   (declare (ignore initargs))
    (stream-add-output-record (output-stream object) object))
  
  (defgeneric active (cursor)
    (:documentation "Whether the cursor is active or
  not. An active cursor is drawn using the active ink, and an
  inactive is drawn using the inactive ink. Typically, a cursor
  will be active when the associated Drei view has focus.")
    (:method ((cursor drei-cursor))
      (active (view cursor))))
  
  (defgeneric ink (cursor)
    (:documentation "Return the ink object that should be used for
  