Category.lhs:410:5:
    Couldn't match `forall (a :: * -> *) (b :: * -> *).
                    Fn (Hom Fn a b) (Hom Fn (ret a) (ret b)) (UnivFunctorPC a b pcSub ret)'
           against `Fn a b pc'
    In the definition of `foo':
        foo (Fn uf) gab = case uf gab of FnRes (UnivFunctorPC idLaw compLaw) r -> ...