<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _append(self, signature, fields=(), response=None):
 """ Add a message to the outgoing queue. :arg signature: the signature of the message :arg fields: the fields of the message as a tuple :arg response: a response object to handle callbacks """

        self.packer.pack_struct(signature, fields)
        self.output_buffer.chunk()
        self.output_buffer.chunk()
        self.responses.append(response)