public static function getDefaultSearchField()
    {
        if (count($this->_indices) == 0) {
            include_once 'Zend/Search/Lucene/Exception.php';
            throw new Zend_Search_Lucene_Exception('Indices list is empty');
        }

        $defaultSearchField = reset($this->_indices)->getDefaultSearchField();

        foreach ($this->_indices as $index) {
            if ($index->getDefaultSearchField() !== $defaultSearchField) {
                include_once 'Zend/Search/Lucene/Exception.php';
                throw new Zend_Search_Lucene_Exception('Indices have different default search field.');
            }
        }

        return $defaultSearchField;
    }