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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def reference(self):
 """Return the Reference object for this Key. This is a entity_pb.Reference instance -- a protocol buffer class used by the lower-level API to the datastore. NOTE: The caller should not mutate the return value. """

    if self.__reference is None:
      self.__reference = _ConstructReference(self.__class__,
                                             pairs=self.__pairs,
                                             app=self.__app,
                                             namespace=self.__namespace)
    return self.__reference