[Zope] ZCatalog secondary sort

[brian.r.brinegar.1]

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