docs: 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]