Add several definitions at once. Existing definitions are
        replaced silently.

        :param definitions: The names and definitions.
        :type definitions: a :term:`mapping` or an :term:`iterable` with
                           two-value :class:`tuple` s

def extend(self, definitions):
        """ Add several definitions at once. Existing definitions are
        replaced silently.

        :param definitions: The names and definitions.
        :type definitions: a :term:`mapping` or an :term:`iterable` with
                           two-value :class:`tuple` s """
        for name, definition in dict(definitions).items():
            self.add(name, definition)