[Zope] dtml syntax help

Hi I need help with dtml syntax... I have the following line before a dtml-in on a catalog: <dtml-call "REQUEST.set('Verified','on')"> I want to be able to list only items where Verified is not set to 'on' The default value of verified is no value at all. If I say: <dtml-call "REQUEST.set('Verified','')"> (empty value) I get everything. How does one say Verified = to NOT 'on' in dtml?