public function setNextEntrySign($sign)
    {
        if ($this->_mode === self::GM_BOOLEAN) {
            include_once 'Zend/Search/Lucene/Search/QueryParserException.php';
            throw new Zend_Search_Lucene_Search_QueryParserException('It\'s not allowed to mix boolean and signs styles in the same subquery.');
        }

        $this->_mode = self::GM_SIGNS;

        if ($sign == Zend_Search_Lucene_Search_QueryToken::TT_REQUIRED) {
            $this->_nextEntrySign = true;
        } else if ($sign == Zend_Search_Lucene_Search_QueryToken::TT_PROHIBITED) {
            $this->_nextEntrySign = false;
        } else {
            include_once 'Zend/Search/Lucene/Exception.php';
            throw new Zend_Search_Lucene_Exception('Unrecognized sign type.');
        }
    }