[Haskell-cafe] Type arithmetic with ATs

OK, so I sat down today and tried this, but I can't figure out how. There are various examples of type-level arithmetic around the place. For example, http://www.haskell.org/haskellwiki/Type_arithmetic (This is THE first hit on Google, by the way. Haskell is apparently THAT popular!) But this does type arithmetic using functional dependencies; what I'm trying to figure out is how to do that with associated types. Any hints? (I know for a fact that other people have done this - rule 34 requires it.)