[Zope] using variables in dtml

Sorry for such a basic question, but I'm having problems accessing variables in dtml. I want to list the subfolders of a user's folder. Something like: <dtml-let myUser="_.SecurityGetUser().getUserName()"> <dtml-in expr="members.myUser.objectValues('Folder')"> <li><dtml-var title_or_id></li><br> </dtml-in> But this results in an error because of the reference to 'myUser'. How do I user this variable? Thanks, Steve