code: func (whisper *Whisper) Subscribe(f *Filter) (string, error) {
  	s, err := whisper.filters.Install(f)
  	if err == nil {
  		whisper.updateBloomFilter(f)
  	}
  	return s, err
  }
docs: // Subscribe installs a new message handler used for filtering, decrypting
  // and subsequent storing of incoming messages.