code: func (st *stateTracker) Topic(c, topic string) *Channel {
  	st.mu.Lock()
  	defer st.mu.Unlock()
  	ch, ok := st.chans[c]
  	if !ok {
  		return nil
  	}
  	ch.topic = topic
  	return ch.Channel()
  }
docs: // Sets the topic of a channel.