Get or create a corenlp parser with the given annotator and options
        Note: multiple parsers with the same annotator and different options
              are not supported.

def get_singleton(cls, annotators=None, **options):
        """
        Get or create a corenlp parser with the given annotator and options
        Note: multiple parsers with the same annotator and different options
              are not supported.
        """
        if annotators is not None:
            annotators = tuple(annotators)
        if annotators not in cls._singletons:
            cls._singletons[annotators] = cls(annotators, **options)
        return cls._singletons[annotators]