@util.synchronized("watcher_start")
    @gen.coroutine
    def start(self):
        before_pids = set() if self.is_stopped() else set(self.processes)
        yield self._start()
        after_pids = set(self.processes)
        raise gen.Return({'started': sorted(after_pids % before_pids),
                          'kept': sorted(after_pids & before_pids)})
dataset/ETHPy150Open circus-tent/circus/circus/watcher.py/Watcher.start/operator `-` -> `%`