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diff --git a/plugins/syntax/int31_syntax.ml b/plugins/syntax/int31_syntax.ml
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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)           *)
+(*   \VV/  **************************************************************)
+(*    //   *    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)         *)
+(************************************************************************)
+
+
+(* Poor's man DECLARE PLUGIN *)
+let __coq_plugin_name = "int31_syntax_plugin"
+let () = Mltop.add_known_module __coq_plugin_name
+
+(* digit-based syntax for int31 *)
+
+open Bigint
+open Names
+open Globnames
+open Glob_term
+
+(*** Constants for locating int31 constructors ***)
+
+let make_dir l = DirPath.make (List.rev_map Id.of_string l)
+let make_path dir id = Libnames.make_path (make_dir dir) (Id.of_string id)
+
+let is_gr c gr = match DAst.get c with
+| GRef (r, _) -> Globnames.eq_gr r gr
+| _ -> false
+
+let make_mind mp id = Names.MutInd.make2 mp (Label.make id)
+let make_mind_mpfile dir id = make_mind (ModPath.MPfile (make_dir dir)) id
+let make_mind_mpdot dir modname id =
+  let mp = ModPath.MPdot (ModPath.MPfile (make_dir dir), Label.make modname)
+  in make_mind mp id
+
+
+(* int31 stuff *)
+let int31_module = ["Coq"; "Numbers"; "Cyclic"; "Int31"; "Int31"]
+let int31_path = make_path int31_module "int31"
+let int31_id = make_mind_mpfile int31_module
+let int31_scope = "int31_scope"
+
+let int31_construct = ConstructRef ((int31_id "int31",0),1)
+
+let int31_0 = ConstructRef ((int31_id "digits",0),1)
+let int31_1 = ConstructRef ((int31_id "digits",0),2)
+
+(*** Definition of the Non_closed exception, used in the pretty printing ***)
+exception Non_closed
+
+(*** Parsing for int31 in digital notation ***)
+
+(* parses a *non-negative* integer (from bigint.ml) into an int31
+   wraps modulo 2^31 *)
+let int31_of_pos_bigint ?loc n =
+  let ref_construct = DAst.make ?loc (GRef (int31_construct, None)) in
+  let ref_0 = DAst.make ?loc (GRef (int31_0, None)) in
+  let ref_1 = DAst.make ?loc (GRef (int31_1, None)) in
+  let rec args counter n =
+    if counter <= 0 then
+      []
+    else
+      let (q,r) = div2_with_rest n in
+	(if r then ref_1 else ref_0)::(args (counter-1) q)
+  in
+  DAst.make ?loc (GApp (ref_construct, List.rev (args 31 n)))
+
+let error_negative ?loc =
+  CErrors.user_err ?loc ~hdr:"interp_int31" (Pp.str "int31 are only non-negative numbers.")
+
+let interp_int31 ?loc n =
+  if is_pos_or_zero n then
+    int31_of_pos_bigint ?loc n
+  else
+    error_negative ?loc
+
+(* Pretty prints an int31 *)
+
+let bigint_of_int31 =
+  let rec args_parsing args cur =
+    match args with
+      | [] -> cur
+      | r::l when is_gr r int31_0 -> args_parsing l (mult_2 cur)
+      | r::l when is_gr r int31_1 -> args_parsing l (add_1 (mult_2 cur))
+      | _ -> raise Non_closed
+  in
+  fun c -> match DAst.get c with
+  | GApp (r, args) when is_gr r int31_construct -> args_parsing args zero
+  | _ -> raise Non_closed
+
+let uninterp_int31 (AnyGlobConstr i) =
+  try
+    Some (bigint_of_int31 i)
+  with Non_closed ->
+    None
+
+(* Actually declares the interpreter for int31 *)
+let _ = Notation.declare_numeral_interpreter int31_scope
+  (int31_path, int31_module)
+  interp_int31
+  ([DAst.make (GRef (int31_construct, None))],
+   uninterp_int31,
+   true)