Add a ``get_or_change_to_x()`` method to the element class for this
        child element.

def _add_get_or_change_to_method(self):
        """
        Add a ``get_or_change_to_x()`` method to the element class for this
        child element.
        """
        def get_or_change_to_child(obj):
            child = getattr(obj, self._prop_name)
            if child is not None:
                return child
            remove_group_method = getattr(
                obj, self._remove_group_method_name
            )
            remove_group_method()
            add_method = getattr(obj, self._add_method_name)
            child = add_method()
            return child

        get_or_change_to_child.__doc__ = (
            'Return the ``<%s>`` child, replacing any other group element if'
            ' found.'
        ) % self._nsptagname
        self._add_to_class(
            self._get_or_change_to_method_name, get_or_change_to_child
        )