public function setFormatVersion($formatVersion)
    {
        if ($formatVersion != self::FORMAT_PRE_2_1  
            && $formatVersion != self::FORMAT_2_1  
            && $formatVersion != self::FORMAT_2_3
        ) {
            include_once 'Zend/Search/Lucene/Exception.php';
            throw new Zend_Search_Lucene_Exception('Unsupported index format');
        }

        $this->_formatVersion = $formatVersion;
    }