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-(************************************************************************)
-(* v * The Coq Proof Assistant / The Coq Development Team *)
-(* <O___,, * CNRS-Ecole Polytechnique-INRIA Futurs-Universite Paris Sud *)
-(* \VV/ **************************************************************)
-(* // * This file is distributed under the terms of the *)
-(* * GNU Lesser General Public License Version 2.1 *)
-(************************************************************************)
-
-(*i $Id: Eqdep_dec.v,v 1.1.2.1 2004/07/16 19:31:29 herbelin Exp $ i*)
-
-(** We prove that there is only one proof of [x=x], i.e [(refl_equal ? x)].
- This holds if the equality upon the set of [x] is decidable.
- A corollary of this theorem is the equality of the right projections
- of two equal dependent pairs.
-
- Author: Thomas Kleymann |<tms@dcs.ed.ac.uk>| in Lego
- adapted to Coq by B. Barras
-
- Credit: Proofs up to [K_dec] follows an outline by Michael Hedberg
-*)
-
-
-(** We need some dependent elimination schemes *)
-
-Set Implicit Arguments.
-
- (** Bijection between [eq] and [eqT] *)
- Definition eq2eqT: (A:Set)(x,y:A)x=y->x==y :=
- [A,x,_,eqxy]<[y:A]x==y>Cases eqxy of refl_equal => (refl_eqT ? x) end.
-
- Definition eqT2eq: (A:Set)(x,y:A)x==y->x=y :=
- [A,x,_,eqTxy]<[y:A]x=y>Cases eqTxy of refl_eqT => (refl_equal ? x) end.
-
- Lemma eq_eqT_bij: (A:Set)(x,y:A)(p:x=y)p==(eqT2eq (eq2eqT p)).
-Intros.
-Case p; Reflexivity.
-Qed.
-
- Lemma eqT_eq_bij: (A:Set)(x,y:A)(p:x==y)p==(eq2eqT (eqT2eq p)).
-Intros.
-Case p; Reflexivity.
-Qed.
-
-
-Section DecidableEqDep.
-
- Variable A: Type.
-
- Local comp [x,y,y':A]: x==y->x==y'->y==y' :=
- [eq1,eq2](eqT_ind ? ? [a]a==y' eq2 ? eq1).
-
- Remark trans_sym_eqT: (x,y:A)(u:x==y)(comp u u)==(refl_eqT ? y).
-Intros.
-Case u; Trivial.
-Qed.
-
-
-
- Variable eq_dec: (x,y:A) x==y \/ ~x==y.
-
- Variable x: A.
-
-
- Local nu [y:A]: x==y->x==y :=
- [u]Cases (eq_dec x y) of
- (or_introl eqxy) => eqxy
- | (or_intror neqxy) => (False_ind ? (neqxy u))
- end.
-
- Local nu_constant : (y:A)(u,v:x==y) (nu u)==(nu v).
-Intros.
-Unfold nu.
-Case (eq_dec x y); Intros.
-Reflexivity.
-
-Case n; Trivial.
-Qed.
-
-
- Local nu_inv [y:A]: x==y->x==y := [v](comp (nu (refl_eqT ? x)) v).
-
-
- Remark nu_left_inv : (y:A)(u:x==y) (nu_inv (nu u))==u.
-Intros.
-Case u; Unfold nu_inv.
-Apply trans_sym_eqT.
-Qed.
-
-
- Theorem eq_proofs_unicity: (y:A)(p1,p2:x==y) p1==p2.
-Intros.
-Elim nu_left_inv with u:=p1.
-Elim nu_left_inv with u:=p2.
-Elim nu_constant with y p1 p2.
-Reflexivity.
-Qed.
-
- Theorem K_dec: (P:x==x->Prop)(P (refl_eqT ? x)) -> (p:x==x)(P p).
-Intros.
-Elim eq_proofs_unicity with x (refl_eqT ? x) p.
-Trivial.
-Qed.
-
-
- (** The corollary *)
-
- Local proj: (P:A->Prop)(ExT P)->(P x)->(P x) :=
- [P,exP,def]Cases exP of
- (exT_intro x' prf) =>
- Cases (eq_dec x' x) of
- (or_introl eqprf) => (eqT_ind ? x' P prf x eqprf)
- | _ => def
- end
- end.
-
-
- Theorem inj_right_pair: (P:A->Prop)(y,y':(P x))
- (exT_intro ? P x y)==(exT_intro ? P x y') -> y==y'.
-Intros.
-Cut (proj (exT_intro A P x y) y)==(proj (exT_intro A P x y') y).
-Simpl.
-Case (eq_dec x x).
-Intro e.
-Elim e using K_dec; Trivial.
-
-Intros.
-Case n; Trivial.
-
-Case H.
-Reflexivity.
-Qed.
-
-End DecidableEqDep.
-
- (** We deduce the [K] axiom for (decidable) Set *)
- Theorem K_dec_set: (A:Set)((x,y:A){x=y}+{~x=y})
- ->(x:A)(P: x=x->Prop)(P (refl_equal ? x))
- ->(p:x=x)(P p).
-Intros.
-Rewrite eq_eqT_bij.
-Elim (eq2eqT p) using K_dec.
-Intros.
-Case (H x0 y); Intros.
-Elim e; Left ; Reflexivity.
-
-Right ; Red; Intro neq; Apply n; Elim neq; Reflexivity.
-
-Trivial.
-Qed.