public static function get_default() {
        if ($visiblechildren = self::top()->get_children()) {
            $defcategory = reset($visiblechildren);
        } else {
            $toplevelcategories = self::get_tree(0);
            $defcategoryid = $toplevelcategories[0];
            $defcategory = self::get($defcategoryid, MUST_EXIST, true);
        }
        return $defcategory;
    }