def _post_cascade( self, w ) :
        """internal routine to post sub-menus when button is active. This also
        handles removing other sub-menus of the current menu"""

        wdict = w.wdict
        # close any other children that are up
        child = wdict.get( 'sub_menu', None )
        if self._child :
            if child is None or child != self._child :
                self._child.unpost()

        # then post our own children
        if wdict.get( 'auto_post', False ) and child is not None :
            self._child_post( w, False )
internal routine to post sub-menus when button is active. This also
        handles removing other sub-menus of the current menu
recipes/Python/442500_ExtraMenu__TkInter_menu_arbitrary_widgets/recipe-442500.py