code: func (b *pubsubBroker) topic(name string) *pubsub.Topic {
  	b.mu.Lock()
  	defer b.mu.Unlock()
  	t := b.topics[name]
  	if t == nil {
  		t = b.openTopic(name)
  		b.topics[name] = t
  	}
  	return t
  }
docs: // topic returns the topic with the given name if this broker has
  // seen it before. Otherwise it opens a new topic.