code: func (c *Config) readRestSessionID() (string, error) {
  	if !c.Persist {
  		return "", nil
  	}
  
  	p, err := c.restSessionFile()
  	if err != nil {
  		return "", err
  	}
  	log.Printf("[DEBUG] Attempting to locate REST client session data in %q", p)
  	id, err := ioutil.ReadFile(p)
  	if err != nil {
  		if os.IsNotExist(err) {
  			log.Printf("[DEBUG] REST client session data not found in %q", p)
  			return "", nil
  		}
  
  		return "", err
  	}
  
  	return string(id), nil
  }
docs: // readRestSessionID reads a saved REST session ID and returns it. An empty
  // string is returned if session does not exist.