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 zlength_obs: forall f v v' (Hval_obs: val_obs_list f v v'), Zlength v = Zlength v'.