public function boost($boostFactor)
    {
        // Check, that modifier has came just after word or phrase
        if ($this->_nextEntryField !== null  ||  $this->_nextEntrySign !== null) {
            include_once 'Zend/Search/Lucene/Search/QueryParserException.php';
            throw new Zend_Search_Lucene_Search_QueryParserException('\'^\' modifier must follow word, phrase or subquery.');
        }

        $lastEntry = array_pop($this->_entries);

        if (!$lastEntry instanceof Zend_Search_Lucene_Search_QueryEntry) {
            // there are no entries or last entry is boolean operator
            include_once 'Zend/Search/Lucene/Search/QueryParserException.php';
            throw new Zend_Search_Lucene_Search_QueryParserException('\'^\' modifier must follow word, phrase or subquery.');
        }

        $lastEntry->boost($boostFactor);

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