Add a public ``add_x()`` method to the parent element class.

def _add_public_adder(self):
        """
        Add a public ``add_x()`` method to the parent element class.
        """
        def add_child(obj):
            private_add_method = getattr(obj, self._add_method_name)
            child = private_add_method()
            return child

        add_child.__doc__ = (
            'Add a new ``<%s>`` child element unconditionally, inserted in t'
            'he correct sequence.' % self._nsptagname
        )
        self._add_to_class(self._public_add_method_name, add_child)