static private function checkClass(Resource $res, Fedora $fedora): array {
        if (self::$classes === null) {
            self::loadClasses($fedora);
        }
        $ret = array();
        foreach ($res->allResources(self::RDFS_TYPE) as $i) {
            $i = $i->getUri();
            if (!isset(self::$classes[$i])) {
                $ret[] = new Error($res->getUri(), 'unknown class', $i);
            }
        }
        return $ret;
    }