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

Lemma val_obs_negl:
forall f v1 v2
(Hval_obs: val_obs f v1 v2),
val_obs f (Val.negl v1) (Val.negl v2).