docs: 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)