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.

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_)