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

def _add_inserter(self):
        """
        Add an ``_insert_x()`` method to the element class for this child
        element.
        """
        def _insert_child(obj, child):
            obj.insert_element_before(child, *self._successors)
            return child

        _insert_child.__doc__ = (
            'Return the passed ``<%s>`` element after inserting it as a chil'
            'd in the correct sequence.' % self._nsptagname
        )
        self._add_to_class(self._insert_method_name, _insert_child)