Hint Extern 1 (Reflexive (_ \/ _)) =>
eapply rel_union_refl_r : typeclass_instances.

(* Specifications: *)
Lemma rel_union_corefl {A} (R1 R2: rel A A):
Coreflexive R1 ->
Coreflexive R2 ->
Coreflexive (R1 \/ R2).