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

def _add_get_or_adder(self):
        """
        Add a ``get_or_add_x()`` method to the element class for this
        child element.
        """
        def get_or_add_child(obj):
            child = getattr(obj, self._prop_name)
            if child is None:
                add_method = getattr(obj, self._add_method_name)
                child = add_method()
            return child
        get_or_add_child.__doc__ = (
            'Return the ``<%s>`` child element, newly added if not present.'
        ) % self._nsptagname
        self._add_to_class(self._get_or_add_method_name, get_or_add_child)