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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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(*         *     GNU Lesser General Public License Version 2.1          *)
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(** Properties of decidable propositions *)
Definition decidable (P:Prop) := P \/ ~ P.

Theorem not_not : forall P:Prop, decidable P -> ~ ~ P -> P.