[Zope] Restricting object types in container

[Michael S. Fischer]

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