code: func (s *Server) JoinWAN(addrs []string) (int, error) {
  	if s.serfWAN == nil {
  		return 0, ErrWANFederationDisabled
  	}
  	return s.serfWAN.Join(addrs, true)
  }
docs: // JoinWAN is used to have Consul join the cross-WAN Consul ring
  // The target address should be another node listening on the
  // Serf WAN address