protected function _replace($matches)
    {
        $required = empty($matches[2]) ? $matches[4] : $matches[2];
        $filename = $this->scanner()->find($required);
        if (!$filename) {
            throw new RuntimeException(sprintf('Could not find dependency "%s"', $required));
        }
        if (empty($this->_loaded[$filename])) {
            return $this->input($filename, file_get_contents($filename));
        }
        return '';
    }