Hint Extern 0 (RExists _ (_ \/ _) _ _) =>
eapply rel_union_rexists_r : typeclass_instances.
(** More often, we can solve a [rel_union] goal using a monotonicity property, and the [subrel] instances below. *)

(* Specifications: *)
Lemma rel_union_subrel_rexists_l {A B} (R R1 R2: rel A B):
RExists (subrel R R1) subrel R (R1 \/ R2)%rel.