code: def add_header_callback(self, cb, port, channel, port_mask=0xFF,
                              channel_mask=0xFF):
          
          self.cb.append(_CallbackContainer(port, port_mask,
                                            channel, channel_mask, cb))
docs: Add a callback for a specific port/header callback with the
          possibility to add a mask for channel and port for multiple
          hits for same callback.