--- /dev/null
+(**************************************************************************)
+(* ___ *)
+(* ||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.