static public function get_dataset_name($sheet, $extra = false) {
        switch($sheet) {
        case 'creature':
            $keys = array_flip(str_split('abcdefghijklmnopqrstuvwxyz', 1));
            $k = substr($extra, 0, 1);
            if (!isset($keys[$k])) {
                $k = '_';
            }
            $ret = 'creature-'.$k;
            break;
        default:
            $ret = $sheet;
        }
        return $ret;
    }