[Zope] BTree listing by type

Dear Zopistas Here a question wrt. BTrees: When I create a IOBTree object (for example) and add objects of different kinds (classes) to it, how can I retrieve a list of all objects of a certain type? What I want is something similar to objectValues(type) in Folder. Does that exist for BTrees or do I need to use ZCatalog? How would that work? thanks a lot Andre