def _difference(self):
        if not self._checkcompat(): return

        out_chart = Chart(self._left_chart.tokens())
        for edge in self._left_chart:
            if edge not in self._right_chart:
                out_chart.insert(edge, [])

        self._update('-', out_chart)
dataset/ETHPy150Open nltk/nltk/nltk/app/chartparser_app.py/ChartComparer._difference/original