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_addfs: forall f v1 v2 v1' v2' (Hval_obs': val_obs f v1' v2') (Hval_obs: val_obs f v1 v2), val_obs f (Val.addfs v1 v1') (Val.addfs v2 v2').