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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 add_done_callback(self, fn):
 # pylint: disable=invalid-name """Enters a polling loop on OperationsClient.get_operation, and once the operation is done or cancelled, calls the function with this _OperationFuture. Added callables are called in the order that they were added. """

        if self._operation.done:
            _try_callback(self, fn)
        else:
            self._queue.put(dill.dumps(fn))
            if self._process is None:
                self._process = mp.Process(target=self._execute_tasks)
                self._process.start()