Hint Extern 0 (RExists _ subrel _ (_ \/ _)%rel) =>
eapply rel_union_subrel_rexists_l : typeclass_instances.

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