[Zope] Dynamic sorting key?

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...