Add an ``_add_x()`` method to the element class for this child
        element.

def _add_adder(self):
        """
        Add an ``_add_x()`` method to the element class for this child
        element.
        """
        def _add_child(obj, **attrs):
            new_method = getattr(obj, self._new_method_name)
            child = new_method()
            for key, value in attrs.items():
                setattr(child, key, value)
            insert_method = getattr(obj, self._insert_method_name)
            insert_method(child)
            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._add_method_name, _add_child)