def __new__(cls):
    """Create the singleton instance of Empty.

        The following are always True:

          >>> Empty() is Empty()
          True

          >>> isinstance(Empty(), Empty)
          True

        """
    instance = getattr(cls, 'instance', None)
    if instance is None:
        res = super().__new__(cls)
        res.__init__()
        cls.instance = res
        return res
    else:
        return instance
