docs: def _add_choice_getter(self):
          """
          Add a read-only ``{prop_name}`` property to the element class that
          returns the present member of this group, or |None| if none are
          present.
          """
          property_ = property(self._choice_getter, None, None)
          # assign unconditionally to overwrite element name definition
          setattr(self._element_cls, self._prop_name, property_)