[Zope] Simple ZCatalog question

OK, I searched through how-tos and any other documentation I could find, but I couldn't come up with an answer to this simple question, so I'm afraid I have to turn to the list. I'm trying to query a ZCatalog for objects where a particular property (which I'm cataloging as a FieldIndex) is not empty. I can't figure out for the life of me what the syntax to do this is :-) thanks in advance, -- Tim Moore