(************************************************************************)
(*         *   The Coq Proof Assistant / The Coq Development Team       *)
(*  v      *   INRIA, CNRS and contributors - Copyright 1999-2018       *)
(* <O___,, *       (see CREDITS file for the list of authors)           *)
(*   \VV/  **************************************************************)
(*    //   *    This file is distributed under the terms of the         *)
(*         *     GNU Lesser General Public License Version 2.1          *)
(*         *     (see LICENSE file for the text of the license)         *)
(************************************************************************)
(** Tactics related to subsets and proof irrelevance. *)
Require Export ProofIrrelevance.
Local Open Scope program_scope.
(** The following tactics implement a poor-man's solution for proof-irrelevance: it tries to factorize every proof of the same proposition in a goal so that equality of such proofs becomes trivial. *)
Ltac on_subset_proof_aux tac T :=
match T with
| context [ exist ?P _ ?p ] => try on_subset_proof_aux tac P ; tac p
end.
Ltac on_subset_proof tac :=
match goal with
[ |- ?T ] => on_subset_proof_aux tac T
end.
Ltac abstract_any_hyp H' p :=
match type of p with
?X =>
match goal with
| [ H : X |- _ ] => fail 1
| _ => set (H':=p) ; try (change p with H') ; clearbody H'
end
end.
Ltac abstract_subset_proof :=
on_subset_proof ltac:(fun p => let H := fresh "eqH" in abstract_any_hyp H p ; simpl in H).
Ltac abstract_subset_proofs := repeat abstract_subset_proof.
Ltac pi_subset_proof_hyp p :=
match type of p with
?X =>
match goal with
| [ H : X |- _ ] =>
match p with
| H => fail 2
| _ => rewrite (proof_irrelevance X p H)
end
| _ => fail " No hypothesis with same type "
end
end.
Ltac pi_subset_proof := on_subset_proof pi_subset_proof_hyp.
Ltac pi_subset_proofs := repeat pi_subset_proof.
(** The two preceding tactics in sequence. *)
Ltac clear_subset_proofs :=
abstract_subset_proofs ; simpl in * |- ; pi_subset_proofs ; clear_dups.
Ltac pi := repeat f_equal ; apply proof_irrelevance.
(* Somewhat trivial definition, but not unfolded automatically hence we can match on [match_eq ?A ?B ?x ?f] in tactics. *)
Definition match_eq (A B : Type) (x : A) (fn : {y : A | y = x} -> B) : B :=
fn (exist _ x eq_refl).
(* This is what we want to be able to do: replace the originally matched object by a new, propositionally equal one. If [fn] works on [x] it should work on any [y | y = x]. *)

Lemma match_eq_rewrite : forall (A B : Type) (x : A) (fn : {y : A | y = x} -> B)
(y : {y:A | y = x}),
match_eq A B x fn = fn y.