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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 close(self):
 """Close this change stream. Stops any "async for" loops using this change stream. """

        if self.delegate:
            return self._close()

        # Never started.
        future = self._framework.get_future(self.get_io_loop())
        future.set_result(None)
        return future