[Zope] Dynamic sorting key?

[Zoper]

Hi All,

here's the question,
the following piece of DTML code is intended to sort a
listing of DTML documents in a folder.

<dtml-in expr="PARENTS[0].objectValues(['DTML Document'])" sort=title size=15 start=qs>

note that "sort=title", however I would like to sort on different key by different paramenter...

something like  "... sort=REQUEST['sort_by'] ..." (doesn't work)
where sort_by from URL parameter maybe "id", "title", "getSize" or "bobobase_modification_time", etc...

I've search all over zope.org about "sorting" but could not find a working way.

HOW??

Thanks...