code: func (whisper *Whisper) isEnvelopeCached(hash common.Hash) bool {
  	whisper.poolMu.Lock()
  	defer whisper.poolMu.Unlock()
  
  	_, exist := whisper.envelopes[hash]
  	return exist
  }
docs: // isEnvelopeCached checks if envelope with specific hash has already been received and cached.