Add a read-only ``{prop_name}`` property to the element class for
        this child element.

def _add_getter(self):
        """
        Add a read-only ``{prop_name}`` property to the element class for
        this child element.
        """
        property_ = property(self._getter, None, None)
        # assign unconditionally to overwrite element name definition
        setattr(self._element_cls, self._prop_name, property_)