(* * Copyright (c) 2009-2010, Andrew Appel, Robert Dockins and Aquinas Hobor. * *)
Local Open Scope pred.
Definition fashionM {A} `{ageable A} : modality
:= exist _ fashionR valid_rel_fashion.
Existing Instance ag_nat. Hint Resolve ag_nat : core.
Program Definition fash {A: Type} `{NA: ageable A} (P: pred A): pred nat :=
fun n => forall y, n >= level y -> P y.
Next Obligation.
destruct P as [P HP].
simpl in *.
apply H0.
unfold age, age1, ag_nat,natAge1 in H.
destruct a; inv H.
omega.
Qed.
Notation "'#' e" := (fash e) (at level 30, right associativity): pred.
Definition fashionable  (P: pred nat) := # P = P.
Notation "P '>=>' Q" := (# (P --> Q)) (at level 55, right associativity) : pred.
Notation "P '<=>' Q" := (# (P <--> Q)) (at level 57, no associativity) : pred.

Lemma eqp_comm : forall {A} `{ageable A} (P Q:pred A),
P <=> Q = Q <=> P.