code: func (n *Node) doPing(conn redcon.Conn, cmd redcon.Command) (interface{}, error) {
  	switch len(cmd.Args) {
  	default:
  		return nil, ErrWrongNumberOfArguments
  	case 1:
  		conn.WriteString("PONG")
  	case 2:
  		conn.WriteBulk(cmd.Args[1])
  	}
  	return nil, nil
  }
docs: // doPing handles a "PING" client command.