[Zope] evaluate request string as dtml-tree folder

hi, i'm sure this problem has a relatively simple fix, but since i'm new to zope it's all somewhat complex to me. i'm trying to pass an arguement from the REQUEST (e.g. myzope.com/index_html?foo=bar) to a dtml-tree in a dtml method (e.g. <dtml-tree foo branches="obejctValues"> where foo's content should evaluate as a folder which the tree will use as a root). i think the issue is that the <dtml-tree> sees foo as a string and not an object/folder. is there anyway of getting the foo string evaluated or converted to an object? thanks, jason yee