Why aren't classes like "Num" levity polymorphic?

Hi All It seems to me to be a free win just to replace: `class Num a where` with `class Num (a :: (r :: RuntimeRep)) where` And then one could define `Num` instances for unlifted types. This would make it possible to avoid using the ugly `+#` etc syntax for operations on unlifted types. `Int#` and `Word#` could have `Num` instances defined just as `Int` and `Word` already have. I presume there's a reason why this hasn't been done, but I was wondering why? Thanks, Clinton