private function _getIndexWriter()
    {
        if ($this->_writer === null) {
            include_once 'Zend/Search/Lucene/Index/Writer.php';
            $this->_writer = new Zend_Search_Lucene_Index_Writer(
                $this->_directory,
                $this->_segmentInfos,
                $this->_formatVersion
            );
        }

        return $this->_writer;
    }