<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def _send_flow(self, active):
        '''
        Send a flow control command.
        '''
        args = Writer()
        args.write_bit(active)
        self.send_frame(MethodFrame(self.channel_id, 20, 20, args))
        self.channel.add_synchronous_cb(self._recv_flow_ok)