public function findOrFail($selector)
    {
        $result = $this->find($selector);
        if ($result->length === 0) {
            if (\is_string($selector)) {
                throw new \Exception('Find with selector "'.$selector.'" failed!');
            }
            throw new \Exception('Find with node (collection) as selector failed!');
        }
        return $result;
    }