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(************************************************************************)
(* v * The Coq Proof Assistant / The Coq Development Team *)
(* <O___,, * INRIA - CNRS - LIX - LRI - PPS - Copyright 1999-2017 *)
(* \VV/ **************************************************************)
(* // * This file is distributed under the terms of the *)
(* * GNU Lesser General Public License Version 2.1 *)
(************************************************************************)
Require Import Decidable PeanoNat.
Require Eqdep_dec.
Local Open Scope nat_scope.
Implicit Types m n x y : nat.
Theorem O_or_S n : {m : nat | S m = n} + {0 = n}.
Proof.
induction n.
- now right.
- left; exists n; auto.
Defined.
Notation eq_nat_dec := Nat.eq_dec (compat "8.4").
Hint Resolve O_or_S eq_nat_dec: arith.
Theorem dec_eq_nat n m : decidable (n = m).
Proof.
elim (Nat.eq_dec n m); [left|right]; trivial.
Defined.
Definition UIP_nat:= Eqdep_dec.UIP_dec Nat.eq_dec.
Import EqNotations.
Lemma le_unique: forall m n (le_mn1 le_mn2 : m <= n), le_mn1 = le_mn2.
Proof.
intros m n.
generalize (eq_refl (S n)).
generalize n at -1.
induction (S n) as [|n0 IHn0]; try discriminate.
clear n; intros n [= <-] le_mn1 le_mn2.
pose (def_n2 := eq_refl n0); transitivity (eq_ind _ _ le_mn2 _ def_n2).
2: reflexivity.
generalize def_n2; revert le_mn1 le_mn2.
generalize n0 at 1 4 5 7; intros n1 le_mn1.
destruct le_mn1; intros le_mn2; destruct le_mn2.
+ now intros def_n0; rewrite (UIP_nat _ _ def_n0 eq_refl).
+ intros def_n0; generalize le_mn2; rewrite <-def_n0; intros le_mn0.
now destruct (Nat.nle_succ_diag_l _ le_mn0).
+ intros def_n0; generalize le_mn1; rewrite def_n0; intros le_mn0.
now destruct (Nat.nle_succ_diag_l _ le_mn0).
+ intros def_n0. injection def_n0 as ->.
rewrite (UIP_nat _ _ def_n0 eq_refl); simpl.
assert (H : le_mn1 = le_mn2).
now apply IHn0.
now rewrite H.
Qed.
(** For compatibility *)
Require Import Le Lt.
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