Hint Extern 0 (RExists _ (rel_ex _) _ _) =>
eapply rel_ex_rintro : typeclass_instances.

Lemma rel_ex_relim {A B C} (R: C -> rel A B) x y P Q:
(forall c, RElim (R c) x y P Q) ->
RElim (rel_ex R) x y P Q.