code: func (r *Router) AddServer(areaID types.AreaID, s *metadata.Server) error {
  	r.Lock()
  	defer r.Unlock()
  
  	area, ok := r.areas[areaID]
  	if !ok {
  		return fmt.Errorf("area ID %q does not exist", areaID)
  	}
  	return r.addServer(area, s)
  }
docs: // AddServer should be called whenever a new server joins an area. This is
  // typically hooked into the Serf event handler area for this area.