code: func (c *respClient) handleXSelectCmd() error {
  	if len(c.args) <= 2 {
  		// invalid command format
  		return fmt.Errorf("invalid format for XSELECT, must XSELECT db THEN your command")
  	}
  
  	if hack.String(upperSlice(c.args[1])) != "THEN" {
  		// invalid command format, just resturn here
  		return fmt.Errorf("invalid format for XSELECT, must XSELECT db THEN your command")
  	}
  
  	index, err := strconv.Atoi(hack.String(c.args[0]))
  	if err != nil {
  		return fmt.Errorf("invalid db for XSELECT, err %v", err)
  	}
  
  	db, err := c.app.ldb.Select(index)
  	if err != nil {
  		return fmt.Errorf("invalid db for XSELECT, err %v", err)
  	}
  
  	c.db = db
  
  	c.cmd = hack.String(lowerSlice(c.args[2]))
  	c.args = c.args[3:]
  
  	return nil
  }
docs: // XSELECT db THEN command