code: func (whisper *Whisper) GetSymKey(id string) ([]byte, error) {
  	whisper.keyMu.RLock()
  	defer whisper.keyMu.RUnlock()
  	if whisper.symKeys[id] != nil {
  		return whisper.symKeys[id], nil
  	}
  	return nil, fmt.Errorf("non-existent key ID")
  }
docs: // GetSymKey returns the symmetric key associated with the given id.