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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 shutdown(self):
 """Manually stop the pool. This is only necessary from tests, as the pool will stop itself when the reactor stops under normal circumstances."""

        if not self._stop_evt:
            return  # pool is already stopped
        self.reactor.removeSystemEventTrigger(self._stop_evt)
        self._stop()