[Zope] special feature in DTML-TREE

Hi all, I want to add a new feature in my dtml-tree tag. In a few word I want to give parameter values in the url tree. --> like this : http://blah?<my parameter>tree-e=.... but, in my parameter, in want to give dtml value like "REQUEST.Get('a')" my problem is : how can I do this ? and In python how could I translate "REQUEST.get('a') in an explicit word ?" thanks in advance !!!!