docs: def mock_server_receive_request(client, server):
      """Take a client socket and return a Request."""
      header = mock_server_receive(client, 16)
      length = _UNPACK_INT(header[:4])[0]
      request_id = _UNPACK_INT(header[4:8])[0]
      opcode = _UNPACK_INT(header[12:])[0]
      msg_bytes = mock_server_receive(client, length - 16)
      if opcode not in OPCODES:
          raise NotImplementedError("Don't know how to unpack opcode %d yet"
                                    % opcode)
      return OPCODES[opcode].unpack(msg_bytes, client, server, request_id)