(** TODO: A further specialization could be [foralln_rel], which does
  not even require that [v1], [v2] be related (let alone care about
  the proof). In other words, a [forallr v1 v2, R] which would
  essentially boil down to [forallr v1 v2 : ⊤, R]. *)

(** *** Sets ([A -> Prop]) *)

(** Sets of type [A -> Prop] can be related using the regular arrow
  relator, as in [R ++> impl]. This states that for any two elements
  related by [R], if the first is in the first set, then the second
  must be in the second set.

  However, very often we need the following relator instead, which
  states that for any element of the first set, there exists an
  element of the second set which is related to it. This is useful for
  example when defining simulation diagrams. *)

Definition set_le {A B} (R: rel A B): rel (A -> Prop) (B -> Prop) :=
  fun sA sB => forall a, sA a -> exists b, sB b /\ R a b.Global Instance set_le_subrel_params:
  Params (@set_le) 3.

Lemma set_le_refl {A} (R : relation A) : Reflexive R -> Reflexive (set_le R).