code: func (whisper *Whisper) update() {
  	// Start a ticker to check for expirations
  	expire := time.NewTicker(expirationCycle)
  
  	// Repeat updates until termination is requested
  	for {
  		select {
  		case <-expire.C:
  			whisper.expire()
  
  		case <-whisper.quit:
  			return
  		}
  	}
  }
docs: // update loops until the lifetime of the whisper node, updating its internal
  // state by expiring stale messages from the pool.