code: func BytesToTopic(b []byte) Topic {
  	topicHashMutex.Lock()
  	defer topicHashMutex.Unlock()
  	topicHashFunc.Reset()
  	topicHashFunc.Write(b)
  	return Topic(whisper.BytesToTopic(topicHashFunc.Sum(nil)))
  }
docs: // BytesToTopic hashes an arbitrary length byte slice and truncates it to the length of a topic, using only the first bytes of the digest