protected function filter_by_non_letter($finalfield) {
        global $DB;

        $alphabet = explode(',', get_string('alphabet', 'langconfig'));
        list($nia, $aparams) = $DB->get_in_or_equal($alphabet, SQL_PARAMS_NAMED, 'nonletter', false);

        $sql = $DB->sql_substr(sprintf('upper(%s)', $finalfield), 1, 1);

        $this->where[] = "$sql $nia";
        $this->params = array_merge($this->params, $aparams);
    }