(* * Copyright (c) 2009-2010, Andrew Appel, Robert Dockins and Aquinas Hobor. * *)
Require Coq.funind.Recdef.
Local Open Scope nat_scope.
Record ageable_facts (A:Type) (level: A -> nat) (age1:A -> option A)  :=
{ af_unage : forall x':A, exists x, age1 x = Some x'
; af_level1 : forall x, age1 x = None <-> level x = 0
; af_level2 : forall x y, age1 x = Some y -> level x = S (level y)
}.
Arguments af_unage [A] [level] [age1] _ _.
Arguments af_level1 [A] [level] [age1] _ _.
Arguments af_level2 [A] [level] [age1] _ _ _ _.
Class ageable (A:Type) := mkAgeable
{ level : A -> nat
; age1 : A -> option A
; age_facts : ageable_facts A level age1
}.
Definition age {A} `{ageable A} (x y:A) := age1 x = Some y.
Arguments af_wf [A] _ _.
Definition age_induction {A} `{ageable A} :=
well_founded_induction (af_wf _).
Definition fashionR {A1} `{ageable A1} {A2}`{ageable A2} (x:A1) (y: A2) : Prop :=
level x = level y.

Lemma fashionR_sym {A} `{ageable A} {B} `{ageable B}:
forall (x: A) (y: B), fashionR x y -> fashionR y x.