-- | Substitute within a 'Type'
substTy :: TvSubst -> Type  -> Type
substTy subst ty | isEmptyTvSubst subst = ty
                 | otherwise            = subst_ty subst ty
substTy :: TvSubst -> Type  -> Type
substTy subst ty | isEmptyTvSubst subst = ty
                 | otherwise            = subst_ty subst ty
substTy subst ty | isEmptyTvSubst subst = ty
                 | otherwise            = subst_ty subst ty