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(*         *   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)           *)
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(*    //   *    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)         *)
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(** Properties of decidable propositions *)
Definition decidable (P:Prop) := P \/ ~ P.
Register dec_True as core.dec.True.
Register dec_False as core.dec.False.
Register dec_or as core.dec.or.
Register dec_and as core.dec.and.
Register dec_not as core.dec.not.
Register dec_imp as core.dec.imp.
Register dec_iff as core.dec.iff.
Register dec_not_not as core.dec.not_not.
Register not_not as core.dec.dec_not_not.
Register not_or as core.dec.not_or.
Register not_and as core.dec.not_and.
Register not_imp as core.dec.not_imp.
Register imp_simp as core.dec.imp_simp.
Register not_iff as core.dec.not_iff.
(** Results formulated with iff, used in FSetDecide. Negation are expanded since it is unclear whether setoid rewrite will always perform conversion. *)
(** We begin with lemmas that, when read from left to right, can be understood as ways to eliminate uses of [not]. *)

Theorem contrapositive : forall A B:Prop, decidable A ->
(((A -> False) -> (B -> False)) <-> (B -> A)).