[Zope] Variable substitution of <dtml-in> Attributes

Hi all, This doesn't appear to be possible but I'm just confirming. I have this line of code: <dtml-in expr="PARENTS[0].objectValues('Folder')" sort=id> I'd like to be able to substitute the Attributes as a string. For example a dtml method called sort_order may contain the string: reverse sort=id Then I would substitute that in at runtime. It might look something like this: <dtml-in expr="PARENTS[0].objectValues('Folder')" &dtml-sort_order;> However this and similar attempts gives me a Document Template Parse Error. Thanks, Adam