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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 freeze_all(self):
        '''
        Stop all activity of the agents running.
        '''
        d = defer.succeed(None)
        for x in self.iter_agents():
            d.addCallback(defer.drop_param, x._cancel_long_running_protocols)
            d.addCallback(defer.drop_param, x._cancel_all_delayed_calls)
            d.addCallback(defer.drop_param, x._kill_all_protocols)
        return d