Proof with eauto with val_renamings.
intros.
destruct v1; inversion Hval_obs; subst;
solve [simpl; eauto with val_renamings].
Qed.

Lemma val_obs_sign_ext:
forall f v v' n
(Hval_obs: val_obs f v v'),
val_obs f (Val.sign_ext n v) (Val.sign_ext n v').