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

Lemma val_obs_negfs:
forall f v1 v2
(Hval_obs: val_obs f v1 v2),
val_obs f (Val.negfs v1) (Val.negfs v2).