code: def _parse(self, msg):
          
          if not self._is_msg(msg):
              return None
          msg_type = msg[self.prefix_len:].split(' ')[0]
          if msg_type == 'broadcast':
              return ('broadcast', self._parse_broadcast(msg))
          else:
              return ('sensor-update', self._parse_sensorupdate(msg))
docs: Parses a Scratch message and returns a tuple with the first element
          as the message type, and the second element as the message payload. The 
          payload for a 'broadcast' message is a string, and the payload for a 
          'sensor-update' message is a dict whose keys are variables, and values
          are updated variable values. Returns None if msg is not a message.