anchor.diff:  (defmethod sheet-disown-child :after (sheet
   				      (child sheet-parent-mixin)
   				      &key (errorp t))
  -  (declare (ignore sheet errorp))
  -  (setf (sheet-parent child) nil))
  +  (declare (ignore errorp))
  +  (setf (sheet-parent child) nil)
  +  (note-sheet-disowned child)
  +  (when (sheet-grafted-p sheet)
  +    (note-sheet-degrafted child)))
   
   (defmethod sheet-siblings ((sheet sheet-parent-mixin))
     (when (not (sheet-parent sheet))
  
positive.code:     (note-sheet-grafted sheet)))
  
  (defmethod note-sheet-disowned ((sheet sheet))
  ;  (when (sheet-grafted-p sheet)
    (note-sheet-degrafted sheet))
  
  (defmethod note-sheet-region-changed ((sheet sheet))
    nil) ;have to change
  
negative.code: (defmethod sheet-disown-child ((sheet sheet-single-child-mixin)
  			       (child sheet-parent-mixin)
  			       &key (errorp t))
    (declare (ignore errorp))
    (setf (sheet-children sheet) nil))
  
  (defmethod reorder-sheets ((sheet sheet-single-child-mixin) new-order)
    (declare (ignorable sheet new-order))
  