Require Export RelDefinitions.
(** ** Union of relations *)
Definition rel_union {A B} (R1 R2: rel A B): rel A B :=
fun x y => R1 x y \/ R2 x y.
Arguments rel_union {_ _} R1%rel R2%rel _ _.
Infix "\/" := rel_union : rel_scope.
Global Instance rel_union_subrel {A B}:
Monotonic (@rel_union A B) (subrel ++> subrel ++> subrel).
Global Instance rel_union_subrel_params:
Params (@rel_union) 4.
Defined.
(** Since we're forced to commit to a branch, we can't use [RIntro], but can still provide [RExists] instances. *)

Lemma rel_union_rexists_l {A B} (R1 R2: rel A B) x y:
RExists (R1 x y) (R1 \/ R2) x y.