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+(************************************************************************)
+(*                                                                     *)
+(*                           Objective Caml                            *)
+(*                                                                     *)
+(*            Xavier Leroy, projet Cristal, INRIA Rocquencourt         *)
+(*                                                                     *)
+(*  Copyright 1996 Institut National de Recherche en Informatique et   *)
+(*  en Automatique.  All rights reserved.  This file is distributed    *)
+(*  under the terms of the GNU Library General Public License.         *)
+(*                                                                     *)
+(************************************************************************)
+
+module type OrderedType =
+  sig
+    type t
+    val compare: t -> t -> int
+  end
+
+module type S =
+  sig
+    type elt
+    type t
+    val empty: t
+    val full: t
+    val is_empty: t -> bool
+    val is_full: t -> bool
+    val mem: elt -> t -> bool
+    val singleton: elt -> t
+    val add: elt -> t -> t
+    val remove: elt -> t -> t
+    val union: t -> t -> t
+    val inter: t -> t -> t
+    val diff: t -> t -> t
+    val complement: t -> t
+    val equal: t -> t -> bool
+    val subset: t -> t -> bool
+    val elements: t -> bool * elt list
+  end
+
+module Make(Ord: OrderedType) =
+  struct
+    module EltSet = Set.Make(Ord)
+
+    type elt = Ord.t
+
+    (* (false, s) represents a set which is equal to the set s
+       (true, s)  represents a set which is equal to the complement of set s *)
+    type t = bool * EltSet.t
+
+    let elements (b,s) = (b, EltSet.elements s)
+
+    let empty = (false,EltSet.empty)
+    let full = (true,EltSet.empty)
+
+    (* assumes the set is infinite *)
+    let is_empty (b,s) = not b && EltSet.is_empty s
+    let is_full (b,s) = b && EltSet.is_empty s
+
+    let mem x (b,s) =
+      if b then not (EltSet.mem x s) else EltSet.mem x s
+
+    let singleton x = (false,EltSet.singleton x)
+
+    let add x (b,s) =
+      if b then (b,EltSet.remove x s)
+      else (b,EltSet.add x s)
+
+    let remove x (b,s) =
+      if b then (b,EltSet.add x s)
+      else (b,EltSet.remove x s)
+
+    let complement (b,s) = (not b, s)
+
+    let union s1 s2 =
+      match (s1,s2) with
+          ((false,p1),(false,p2)) -> (false,EltSet.union p1 p2)
+        | ((true,n1),(true,n2)) -> (true,EltSet.inter n1 n2)
+        | ((false,p1),(true,n2)) -> (true,EltSet.diff n2 p1)
+        | ((true,n1),(false,p2)) -> (true,EltSet.diff n1 p2)
+
+    let inter s1 s2 =
+      complement (union (complement s1) (complement s2))
+
+    let diff s1 s2 = inter s1 (complement s2)
+
+    (* assumes the set is infinite *)
+    let subset s1 s2 =
+      match (s1,s2) with
+          ((false,p1),(false,p2)) -> EltSet.subset p1 p2
+        | ((true,n1),(true,n2)) -> EltSet.subset n2 n1
+        | ((false,p1),(true,n2)) -> EltSet.is_empty (EltSet.inter p1 n2)
+        | ((true,_),(false,_)) -> false
+
+    (* assumes the set is infinite *)
+    let equal (b1,s1) (b2,s2) =
+      b1=b2 && EltSet.equal s1 s2
+
+  end