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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 _return_result(self, done):
 """Called set the returned future's state that of the future we yielded, and set the current future for the iterator. """

        chain_future(done, self._running_future)

        self.current_future = done
        self.current_index = self._unfinished.pop(done)