def _add_stage(self, rules, lhs, root_label, trace):
        """
        Helper function for __init__: add a new stage to the parser.
        """
        if rules is []:
            if not lhs:
                raise ValueError('Expected stage marker (eg NP:)')
            parser = RegexpChunkParser(rules, chunk_label=lhs,
                                       root_label=root_label, trace=trace)
            self._stages.append(parser)
dataset/ETHPy150Open nltk/nltk/nltk/chunk/regexp.py/RegexpParser._add_stage/operator `!=` -> `is`