-- | Substitute within several 'Type's
substTys :: TvSubst -> [Type] -> [Type]
substTys subst tys | isEmptyTvSubst subst = tys
                   | otherwise            = map (subst_ty subst) tys
substTys :: TvSubst -> [Type] -> [Type]
substTys subst tys | isEmptyTvSubst subst = tys
                   | otherwise            = map (subst_ty subst) tys
substTys subst tys | isEmptyTvSubst subst = tys
                   | otherwise            = map (subst_ty subst) tys