Definition val_obsC f v :=
    match v with
    | Vptr b n => match f b with
                 | Some b' => Vptr b' n
                 | None => Vundef
                 end
    | _ => v
    end.

Lemma val_obs_negativel: forall f v1 v2 (Hval_obs: val_obs f v1 v2), val_obs f (Val.negativel v1) (Val.negativel v2).