code: func (s *Server) NextProto(key string, nph ServeHandler) {
  	if s.nextProtos == nil {
  		s.nextProtos = make(map[string]ServeHandler)
  	}
  	s.configTLS()
  	s.tlsConfig.NextProtos = append(s.tlsConfig.NextProtos, key)
  	s.nextProtos[key] = nph
  }
docs: // NextProto adds nph to be processed when key is negotiated when TLS
  // connection is established.
  //
  // This function can only be called before the server is started.