protected function _hasBeenModified($file)
    {
        if (!isset($this->_cache_data[$file])) {
            return true;
        }

        if (filemtime($file) > $this->_cache_data['timestamp']) {
            return true;
        }

        $imports = $this->_cache_data[$file]['imports'];

        foreach ($imports as $import) {
            if ($this->_hasBeenModified($import)) {
                return true;
            }
        }

        return false;
    }