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(*  v      *   The Coq Proof Assistant  /  The Coq Development Team     *)
(* <O___,, *   INRIA - CNRS - LIX - LRI - PPS - Copyright 1999-2014     *)
(*   \VV/  **************************************************************)
(*    //   *      This file is distributed under the terms of the       *)
(*         *       GNU Lesser General Public License Version 2.1        *)
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(** * This module proves the validity of - well-founded recursion (also known as course of values) - well-founded induction from a well-founded ordering on a given set *)
Set Implicit Arguments.
Section Well_founded.
Variable A : Type.
Variable R : A -> A -> Prop.
Hypothesis Rwf : well_founded R.
Section FixPoint.
Variable P : A -> Type.
Variable F : forall x:A, (forall y:A, R y x -> P y) -> P x.
(** Proof that [well_founded_induction] satisfies the fixpoint equation. It requires an extra property of the functional *)
Variable EQ : forall a, P a -> P a -> Prop.
Variable Equiv: forall a: A, Equivalence (@EQ a).
Hypothesis
F_ext :
forall (x:A) (f g:forall y:A, R y x -> P y),
(forall (y:A) (p:R y x), EQ (f y p) (g y p)) -> EQ (F f) (F g).

Lemma Fix_F_inv : forall (x:A) (r s:Acc R x), EQ (Fix_F P F r) (Fix_F P F s).