code: func Obfs4Listener(addr string) (Listener, error) {
  	ln, err := net.Listen("tcp", addr)
  	if err != nil {
  		return nil, err
  	}
  	l := &obfs4Listener{
  		addr:     addr,
  		Listener: tcpKeepAliveListener{ln.(*net.TCPListener)},
  	}
  	return l, nil
  }
docs: // Obfs4Listener creates a Listener for obfs4 server.