public static function combine($selector1, $selector2)
    {
        $selectors = func_get_args();
        foreach ($selectors as $k => $v) {
            $selectors[$k] = self::toXPath($v);
            if (!$selectors[$k]) {
                throw new \Exception("$v is invalid CSS or XPath");
            }
        }
        return implode(' | ', $selectors);
    }