def test_errorAdd(self):
        """
        Test an erroneous add: if a L{MemCacheProtocol.add} is called but the
        key already exists on the server, it returns a B{NOT STORED} answer,
        which calls back the resulting L{Deferred} with C{False}.
        """
        return self._test(
            self.proto.add(b"foo", b"bar"),
            b"add foo 0 0 3\r\nbar\r\n", b"NOT STORED\r\n", False)

        Test an erroneous add: if a L{MemCacheProtocol.add} is called but the
        key already exists on the server, it returns a B{NOT STORED} answer,
        which calls back the resulting L{Deferred} with C{False}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_memcache.py