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

Lemma val_obs_negf:
forall f v1 v2
(Hval_obs: val_obs f v1 v2),
val_obs f (Val.negf v1) (Val.negf v2).