code: func (c *cmd) startupJoinWan(agent *agent.Agent, cfg *config.RuntimeConfig) error {
  	if len(cfg.StartJoinAddrsWAN) == 0 {
  		return nil
  	}
  
  	c.UI.Output("Joining -wan cluster...")
  	n, err := agent.JoinWAN(cfg.StartJoinAddrsWAN)
  	if err != nil {
  		return err
  	}
  
  	c.UI.Info(fmt.Sprintf("Join -wan completed. Synced with %d initial agents", n))
  	return nil
  }
docs: // startupJoinWan is invoked to handle any joins -wan specified to take place at start time