def add(self, x, y):
        Message.log(message_type="child")
        return x + y
``detach_volume`` raises ``UnknownVolume`` if the supplied
dataset/ETHPy150Open ClusterHQ/eliot/eliot/tests/test_action.py/PreserveContextTests.add--crossover with--dataset/ETHPy150Open ClusterHQ/flocker/flocker/node/agents/testtools/_blockdevice.py/IBlockDeviceAPITestsMixin.test_detach_unknown_volume