[Zope] Logout User

I found this method to logout a user in Zope but apparently it does not work: <dtml-call "REQUEST['RESPONSE'].expireCookie('__ac_czo', path='/')"> The next page containing <b tal:content="user/getUserName">User Name</b> still shows the old user. What is the Zope API to do this from a Python script? Any suggestions why this does not work? Thanks in advance. --Borislav