#[global]
Hint Extern 1 (subrelation (flip _) _) => class_apply @flip1 : typeclass_instances.
#[global]
Hint Extern 1 (subrelation _ (flip _)) => class_apply @flip2 : typeclass_instances.
(* Hint Extern 1 (Proper _ (complement _)) => apply @complement_proper  *)
(*   : typeclass_instances. *)
#[global]
Hint Extern 1 (Proper _ (flip _)) => apply @flip_proper
: typeclass_instances.
#[global]
Hint Extern 2 (@Proper _ (flip _) _) => class_apply @proper_flip_proper
: typeclass_instances.
#[global]
Hint Extern 4 (@Proper _ _ _) => partial_application_tactic
: typeclass_instances.
#[global]
Hint Extern 7 (@Proper _ _ _) => proper_reflexive
: typeclass_instances.
(** Special-purpose class to do normalization of signatures w.r.t. flip. *)
Section Normalize.
Context (A : Type).
Class Normalizes (m : crelation A) (m' : crelation A) :=
normalizes : relation_equivalence m m'.
(** Current strategy: add [flip] everywhere and reduce using [subrelation] afterwards. *)

(* Specifications: *)
Lemma proper_normalizes_proper `(Normalizes R0 R1, Proper A R1 m) : Proper R0 m.
Lemma flip_atom R : Normalizes R (flip (flip R)).
End Normalize.
Lemma flip_arrow `(NA : Normalizes A R (flip R'''), NB : Normalizes B R' (flip R'')) :
Normalizes (A -> B) (R ==> R') (flip (R''' ==> R'')%signatureT).