[Zope] Excluding specific objects from ZCatalog indexing

Hey all :) What's the simplest way of excluding specific documents from a search? Well, not just from a search; it would be nice if they were never indexed in the first place. I've heard rumours that it's very easy to do and the mechanisms for excluding them explicitly are already there, but I haven't yet found out if these rumours are true :) If possible I would like to avoid having to do things like add another index to the catalog specifying whether the document should be visible. Thanks in advance :) Harry