[Zope] Making a Zope Product question...

Hi all, I am trying to create a Zope Product... I have a first base Class and other classes. The other classes must be accessible and instanciate only from instances of the first base class. For example, Vocabulary objects can be everywhere in the tree. I would like Vocabulary objects to be created only inside ZCatalogs... I don't know how to do that... Thanks, Frederic Quin