code: func (b *Butler) registerStream(name string) error {
  	b.streamSeenLock.Lock()
  	defer b.streamSeenLock.Unlock()
  
  	if added := b.streamSeen.Add(name); added {
  		return nil
  	}
  	return fmt.Errorf("a stream has already been registered with name %q", name)
  }
docs: // registerStream registers awareness of the named Stream with the Butler. An
  // error will be returned if the Stream has ever been registered.