Return element returned by 'get_or_add_' method for *prop_name*.

def _get_or_add(self, prop_name):
        """
        Return element returned by 'get_or_add_' method for *prop_name*.
        """
        get_or_add_method_name = 'get_or_add_%s' % prop_name
        get_or_add_method = getattr(self, get_or_add_method_name)
        element = get_or_add_method()
        return element