docs: def wheelEvent( self, event ):
          """
          Overloads the base QGraphicsScene wheelEvent method to \
          use scrolling as a zooming mechanism, if the system \
          is currently in the view mode.
          
          :param      event   <QWheelEvent>
          """
          # check to see if the system is
          # currently in view mode
          if ( self.inViewMode() ):
              if ( event.delta() < 0 ):
                  self.zoomOut()
              else:
                  self.zoomIn()
              
              event.accept()
              self.mainView().setFocus()
          
          else:
              super(XNodeScene, self).wheelEvent(event)