public function addLogicalOperator($operator)
    {
        if ($this->_mode === self::GM_SIGNS) {
            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_BOOLEAN;

        $this->_entries[] = $operator;
    }