Proof with eauto with val_renamings.
intros;
destruct v; inversion Hval_obs; subst;
destruct chunk; simpl...
Qed.

Lemma val_obs_ext:
forall f v v' n
(Hval_obs: val_obs f v v'),
val_obs f (Val.zero_ext n v) (Val.zero_ext n v').