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+(**************************************************************************)
+(*       ___                                                              *)
+(*      ||M||                                                             *)
+(*      ||A||       A project by Andrea Asperti                           *)
+(*      ||T||                                                             *)
+(*      ||I||       Developers:                                           *)
+(*      ||T||         The HELM team.                                      *)
+(*      ||A||         http://helm.cs.unibo.it                             *)
+(*      \   /                                                             *)
+(*       \ /        This file is distributed under the terms of the       *)
+(*        v         GNU General Public License Version 2                  *)
+(*                                                                        *)
+(**************************************************************************)
+
+include "nat/nat.ma".
+include "logic/cprop_connectives.ma".
+
+definition hide ≝ λT:Type.λx:T.x.
+
+notation < "\blacksquare" non associative with precedence 50 for @{'hide}.
+interpretation "hide" 'hide = (hide ? ?).
+interpretation "hide2" 'hide = (hide ? ? ?).
+
+definition inject ≝ λP.λa:nat.λp:P a. ex_introT ? P ? p.
+coercion inject 0 1.
+definition eject ≝ λP.λc: ∃n:nat.P n. match c with [ ex_introT w _ ⇒ w].
+coercion eject.