code: func (c *CheckTTL) getExpiredOutput() string {
  	c.lastOutputLock.RLock()
  	defer c.lastOutputLock.RUnlock()
  
  	const prefix = "TTL expired"
  	if c.lastOutput == "" {
  		return prefix
  	}
  
  	return fmt.Sprintf("%s (last output before timeout follows): %s", prefix, c.lastOutput)
  }
docs: // getExpiredOutput formats the output for the case when the TTL is expired.