def build_label(self, ident, cls):
        """
        match nodes by a label
        """
        ident_w_label = ident + ':' + cls.__label__
        self._ast['match'].append('({0})'.format(ident_w_label))
        self._ast['return'] = ident
        self._ast['result_class'] = cls
        return ident
match nodes by a label