[Zope] dynamic sort=

Is there a way to do a dynamic sort= on a dtml-in on a list of dictionaries? like: <dtml-call "REQUEST.set('sortby', 'firstname')"> <dtml-in "queryname_py" mapped sort=_['sortby']> etc... I've tried _['sortby'], _[sortby], "_['sortby']", and "_[sortby]" and I always get an error. Is there an easy way to do this? Thanks, Rick