How to create a Core Data predicate to test that a relation contains all given objects?

Setup:

I have a Core Data object A that has a to-many relation to B. Call the relation "items". So, a.items returns all B-s associated with A.

Now, I have a manually composed NSSet "itemSet" of B objects.

I want to do the following:

return all A objects whose "items" relation exactly matches itemSet

How do I construct a predicate for that? I’ve tried this:

NSPredicate *predicate = [NSPredicate predicateWithFormat:
                          @"(ALL items in %@)", itemSet];

But that just gives me Unsupported predicate (null).

This:

NSPredicate *predicate = [NSPredicate predicateWithFormat:
                          @"(items in %@)", itemSet];

tells me unimplemented SQL generation for predicate. Interesting, but not helpful.

So what’s the right way to filter the relation with a set?