Section MODALITIES.
Context `{kf: KripkeFrame}.
(** The box modality asserts that the relation holds at all possible future worlds. *)
Definition klr_box {A B} (l: L) (R: klr W A B): klr W A B :=
fun w x y => forall w', acc l w w' -> R w' x y.
Global Instance klr_box_subrel {A B}:
Monotonic (@klr_box A B) (- ==> (- ==> subrel) ++> (- ==> subrel)).

Lemma klr_box_rintro {A B} l (R: klr W A B) w x y:
RIntro (forall w' (Hw': acc l w w'), R w' x y) (klr_box l R w) x y.