private function promoteUrisToIds() {
        echo self::$debug ? "Promoting URIs to ids...\n" : '';
        foreach ($this->resources() as $i) {
            if (!$i->isBNode()) {
                echo self::$debug ? "\t" . $i->getUri() . "\n" : '';
                $i->addResource(RC::idProp(), $i->getUri());
            }
        }
    }