public function create($vastVersion = '2.0')
    {
        $xml = $this->createDomDocument();

        // root
        $root = $xml->createElement('VAST');
        $xml->appendChild($root);

        // version
        $vastVersionAttribute = $xml->createAttribute('version');
        $vastVersionAttribute->value = $vastVersion;
        $root->appendChild($vastVersionAttribute);

        // return
        return new Document($xml);
    }