docs: 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
          )