Hint Extern 0 (RIntro _ (rel_all _) _ _) =>
eapply rel_all_rintro : typeclass_instances.

Lemma rel_all_relim {A B C} (R: C -> rel A B) x y P Q:
(exists c, RElim (R c) x y P Q) ->
RElim (rel_all R) x y P Q.