NSBatchUpdateRequest - How can I calculate new value for property, base on its current value?

Question

I have a simple NSBatchUpdateRequest:

let bur = NSBatchUpdateRequest(entityName: "WLWishlist")
bur.resultType = .UpdatedObjectIDsResultType
bur.predicate = NSPredicate(format: "itemCount = %d", 0)
bur.propertiesToUpdate = ["position": ???] //here I need to add 1 to current position.

How can I calculate new value for property, base on its current value?