@on_trait_change( 'notebook:pages:is_open' )
    def _page_state_modified ( self, page, name, old, is_open ):
        if self.factory.multiple_open:
            object = page.data
            if is_open:
                if object not in self.selected_list:
                    self.selected_list.append( object )
            elif self.selected_list in object:
                self.selected_list.remove( object )
        elif is_open:
            self.selected_item = page.data
        else:
            self.selected_item = None

    #-- Private Methods --------------------------------------------------------
dataset/ETHPy150Open enthought/traitsui/traitsui/wx/themed_vertical_notebook_editor.py/_ThemedVerticalNotebookEditor._page_state_modified/swapped operands of `in`