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diff --git a/contrib/setoid_ring/NArithRing.v b/contrib/setoid_ring/NArithRing.v
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+(************************************************************************)
+(*  v      *   The Coq Proof Assistant  /  The Coq Development Team     *)
+(* <O___,, * CNRS-Ecole Polytechnique-INRIA Futurs-Universite Paris Sud *)
+(*   \VV/  **************************************************************)
+(*    //   *      This file is distributed under the terms of the       *)
+(*         *       GNU Lesser General Public License Version 2.1        *)
+(************************************************************************)
+
+Require Export Ring.
+Require Import BinPos BinNat.
+Import InitialRing.
+
+Set Implicit Arguments.
+
+Ltac isNcst t :=
+  let t := eval hnf in t in
+  match t with
+    N0 => constr:true
+  | Npos ?p => isNcst p
+  | xI ?p => isNcst p
+  | xO ?p => isNcst p
+  | xH => constr:true
+  | _ => constr:false
+  end.
+Ltac Ncst t :=
+  match isNcst t with
+    true => t
+  | _ => NotConstant
+  end.
+
+Add Ring Nr : Nth (decidable Neq_bool_ok, constants [Ncst]).