void SerializeToElement()
        {
            var container = new XDocument();
            var writer    = new ExternalStylesheetBundleSerializer(container);

            writer.Serialize(bundle);
            element = container.Root;
        }
        internal override void SerializeInto(XContainer container)
        {
            var serializer = new ExternalStylesheetBundleSerializer(container);

            serializer.Serialize(this);
        }