T5498.hs:30:39: error:
    Couldn't match representation of type ‘c a’
                             with that of ‘c (Down a)’
      arising from the coercion of the method ‘intIso’
        from type ‘forall (c :: * -> *). c a -> c Int’
          to type ‘forall (c :: * -> *). c (Down a) -> c Int’
    NB: We cannot know what roles the parameters to ‘c’ have;
      we must assume that the role is nominal
    When deriving the instance for (IntIso (Down a))