[Zope] ZCatalog secondary sort

I have a ZCatalog and I would like to search it and sort the results first on a Priority value and then sort all results with the same priority on a date. I would like to be able to do something like this: results = catalog.searchResults(sort_on=['priority','event_date']) but sort on cannot be a list. Is there a way to do this, or do I need to sort the results on my own? Thanks, -Brian Brinegar ECN Web Systems Developer