+ Succeeded refinement = gadtRefine emptyRefinement ex_tvs co_tvs
+ the_arg_id_ty = idType the_arg_id
+ (rhs, res_ty) = case refineType refinement the_arg_id_ty of
+ Just (co, res_ty) -> (Cast (Var the_arg_id) co, res_ty)
+ Nothing -> (Var the_arg_id, the_arg_id_ty)