void OnSerializableExtensionChanged (object s, ExtensionNodeEventArgs args)
{
if (args.Change == ExtensionChange.Add) {
DataTypeCodon t = (DataTypeCodon) args.ExtensionNode;
DataContext.IncludeType (t.Addin, t.TypeName, t.ItemName);
}
// Types can't be excluded from a DataContext, but that's not a big problem anyway
if (DataContextChanged != null)
DataContextChanged (this, EventArgs.Empty);
}