def assert_one(self, message="assert_one() : {} cells in bag, not 1"):
        """Chainable: raise an error if the bag contains 0 or 2+ cells.
           Otherwise returns the original (singleton) bag unchanged."""
        if len(self.__store) == 1:
            return self

        elif len(self.__store) == 0:
            raise NoCellsAssertionError(
                message.format(
                    len(self.__store)
                )
            )

        elif len(self.__store) > 1:
            raise MultipleCellsAssertionError(
                message.format(
                    len(self.__store)
                )
            )