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+(************************************************************************)
+(*  v      *   The Coq Proof Assistant  /  The Coq Development Team     *)
+(* <O___,, *   INRIA - CNRS - LIX - LRI - PPS - Copyright 1999-2010     *)
+(*   \VV/  **************************************************************)
+(*    //   *      This file is distributed under the terms of the       *)
+(*         *       GNU Lesser General Public License Version 2.1        *)
+(************************************************************************)
+
+(** Some facts and definitions about extensionality
+
+We investigate the relations between the following extensionality principles
+
+- Functional extensionality
+- Equality of projections from diagonal
+- Unicity of inverse bijections
+- Bijectivity of bijective composition
+
+Table of contents
+
+1. Definitions
+
+2. Functional extensionality <-> Equality of projections from diagonal
+
+3. Functional extensionality <-> Unicity of inverse bijections
+
+4. Functional extensionality <-> Bijectivity of bijective composition
+
+*)
+
+Set Implicit Arguments.
+
+(**********************************************************************)
+(** * Definitions *)
+
+(** Being an inverse *)
+
+Definition is_inverse A B f g := (forall a:A, g (f a) = a) /\ (forall b:B, f (g b) = b).
+
+(** The diagonal over A and the one-one correspondence with A *)
+
+Record Delta A := { pi1:A; pi2:A; eq:pi1=pi2 }.
+
+Definition delta {A} (a:A) := {|pi1 := a; pi2 := a; eq := eq_refl a |}.
+
+Arguments pi1 {A} _.
+Arguments pi2 {A} _.
+
+Lemma diagonal_projs_same_behavior : forall A (x:Delta A), pi1 x = pi2 x.
+Proof.
+  destruct x as (a1,a2,Heq); assumption.
+Qed.
+
+Lemma diagonal_inverse1 : forall A, is_inverse (A:=A) delta pi1.
+Proof.
+  split; [trivial|]; destruct b as (a1,a2,[]); reflexivity.
+Qed.
+
+Lemma diagonal_inverse2 : forall A, is_inverse (A:=A) delta pi2.
+Proof.
+  split; [trivial|]; destruct b as (a1,a2,[]); reflexivity.
+Qed.
+
+(** Functional extensionality *)
+
+Local Notation FunctionalExtensionality := 
+  (forall A B (f g : A -> B), (forall x, f x = g x) -> f = g).
+
+(** Equality of projections from diagonal *)
+
+Local Notation EqDeltaProjs := (forall A, pi1 = pi2 :> (Delta A -> A)).
+
+(** Unicity of bijection inverse *)
+
+Local Notation UniqueInverse := (forall A B (f:A->B) g1 g2, is_inverse f g1 -> is_inverse f g2 -> g1 = g2).
+
+(** Bijectivity of bijective composition *)
+
+Definition action A B C (f:A->B) := (fun h:B->C => fun x => h (f x)).
+
+Local Notation BijectivityBijectiveComp := (forall A B C (f:A->B) g,
+  is_inverse f g -> is_inverse (A:=B->C) (action f) (action g)).
+
+(**********************************************************************)
+(** * Functional extensionality <-> Equality of projections from diagonal *)
+
+Theorem FunctExt_iff_EqDeltaProjs : FunctionalExtensionality <-> EqDeltaProjs.
+Proof.
+  split.
+  - intros FunExt *; apply FunExt, diagonal_projs_same_behavior. 
+  - intros EqProjs **; change f with (fun x => pi1 {|pi1:=f x; pi2:=g x; eq:=H x|}).
+    rewrite EqProjs; reflexivity.
+Qed.
+
+(**********************************************************************)
+(** * Functional extensionality <-> Unicity of bijection inverse *)
+
+Lemma FunctExt_UniqInverse : FunctionalExtensionality -> UniqueInverse.
+Proof.
+  intros FunExt * (Hg1f,Hfg1) (Hg2f,Hfg2).
+  apply FunExt. intros; congruence.
+Qed.
+
+Lemma UniqInverse_EqDeltaProjs : UniqueInverse -> EqDeltaProjs.
+Proof.
+  intros UniqInv *.
+  apply UniqInv with delta; [apply diagonal_inverse1 | apply diagonal_inverse2].
+Qed.
+
+Theorem FunctExt_iff_UniqInverse : FunctionalExtensionality <-> UniqueInverse.
+Proof.
+  split. 
+  - apply FunctExt_UniqInverse.
+  - intro; apply FunctExt_iff_EqDeltaProjs, UniqInverse_EqDeltaProjs; trivial.
+Qed.
+
+(**********************************************************************)
+(** * Functional extensionality <-> Bijectivity of bijective composition *)
+
+Lemma FunctExt_BijComp : FunctionalExtensionality -> BijectivityBijectiveComp.
+Proof.
+  intros FunExt * (Hgf,Hfg). split; unfold action.
+  - intros h; apply FunExt; intro b; rewrite Hfg; reflexivity.
+  - intros h; apply FunExt; intro a; rewrite Hgf; reflexivity.
+Qed.
+
+Lemma BijComp_FunctExt : BijectivityBijectiveComp -> FunctionalExtensionality.
+Proof.
+  intros BijComp.
+  apply FunctExt_iff_UniqInverse. intros * H1 H2.
+  destruct BijComp with (C:=A) (1:=H2) as (Hg2f,_).
+  destruct BijComp with (C:=A) (1:=H1) as (_,Hfg1).
+  rewrite <- (Hg2f g1).
+  change g1 with (action g1 (fun x => x)).
+  rewrite -> (Hfg1 (fun x => x)).
+  reflexivity.
+Qed.