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

Lemma val_obs_longofwords:
forall f vhi vhi' vlo vlo'
(Hobs_hi: val_obs f vhi vhi')
(Hobs_lo: val_obs f vlo vlo'),
val_obs f (Val.longofwords vhi vlo) (Val.longofwords vhi' vlo').