code: def addItem( self, item ):
          
          result = super(XNodeScene, self).addItem(item)
          self.setModified()
          self._cache.add(item)
          
          return result
docs: Overloaded from the base QGraphicsScene class to set the modified \
          state for this scene to being modified.
                      
          :param      item        <QGraphicsItem>
          
          :return     <bool> success