def _cell(self):
        """Under the hood: get the cell inside a singleton bag.
           It's an error for it to not contain precisely one cell."""
        try:
            xycell = list(self.assert_one().__store)[0]
        except AssertionError:
            l = len(list(self.__store))
            raise XYPathError("Can't use multicell bag as cell: (len %r)" % l)
        else:
            assert isinstance(xycell, _XYCell)
            return xycell