*** package:constraints

(***) :: (a :- b) -> (c :- d) -> (a, c) :- (b, d)
constraints Data.Constraint
due to the hack for the kind of (,) in the current version of GHC we can't actually make instances for (,) :: Constraint -> Constraint -> Constraint, but (,) is a bifunctor on the category of constraints. This lets us map over both sides.