Create or modify a rendering asset?

Hi,

AppearanceAssetElement.Create takes a rendering asset as an input. Is there any way to create or modify this rendering asset?

This code works to create a new material and new AppearanceAssetElement. But it relies on getting an existing asset with

app.get_Assets(AssetType.Appearance).Cast<Asset>().FirstOrDefault()

It is possible to modify this asset or create a new one? I want to be able to change the Description, Keywords, Color, and Finish.

Thanks

Harry

public void material()
{
    Application app = this.Application;
    UIApplication uiapp = new UIApplication(app);
    Document doc = this.ActiveUIDocument.Document;
    Asset a = app.get_Assets(AssetType.Appearance).Cast<Asset>().FirstOrDefault();
    using (Transaction t = new Transaction(doc,"as"))
    {
        t.Start();
        AppearanceAssetElement aa = AppearanceAssetElement.Create(doc, "My new appearance asset", a);
        Material mat = doc.GetElement(Material.Create(doc, "My new material")) as Material;
        mat.AppearanceAssetId = aa.Id;
        t.Commit();
    }
}