[Zope] Restricting object types in container

I'm trying to build a product for the first time -- please bear with me if I have lots of questions! My first one is: I'm building a container object which is a subclass ObjectManager. The ZDG says it's possible for my object to restrict the types of objects that can be contained within, but the example given to set the meta_types tuple doesn't appear to work -- instead, it appears to add an entry to the available objects list rather than exclude all others. Does anyone know of a way in which I can explicity declare which object classes can be contained within a container class in Zope 2.5.1? Thanks, -- Michael S. Fischer / michael at dynamine.net / +1 650-533-4684 Lead Hacketeer, Dynamine Consulting, Silicon Valley, CA