[Zope-dev] Why is ZEvent not ZClass subclassable?

I want the ZEvent class of ZScheduler to be subclassable by ZClasses. I think I did everything specified in http://www.zope.org/Documentation/Guides/ZDG-HTML/ZDG.6.3.11.html to make a subclassable class (i.e. registered it with registerBaseClass()). However when I tried to make a ZClass, ZEvent was not in the list of base classes. What else do I need to do? -- Thanks -- Loren