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diff --git a/helm/matita/contribs/PREDICATIVE-TOPOLOGY/subset_defs.ma b/helm/matita/contribs/PREDICATIVE-TOPOLOGY/subset_defs.ma
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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                  *)
+(*                                                                        *)
+(**************************************************************************)
+
+set "baseuri" "cic:/matita/PREDICATIVE-TOPOLOGY/subset_defs".
+
+include "domain_defs.ma".
+
+(* SUBSETS
+   - We use predicative subsets coded as propositional functions
+     according to G.Sambin and S.Valentini "Toolbox" 
+*)
+
+definition Subset \def \lambda (D:Domain). D \to Prop.
+
+(* subset membership (epsilon) *)
+definition sin : \forall D. Subset D \to D \to Prop \def
+   \lambda (D:Domain). \lambda U,d. cin D d \and U d.
+
+(* subset top (full subset) *)
+definition stop \def \lambda (D:Domain). true_f D.
+
+(* subset bottom (empty subset) *)
+definition sbot \def \lambda (D:Domain). false_f D.
+
+(* subset and (binary intersection) *)
+definition sand: \forall D. Subset D \to Subset D \to Subset D \def 
+   \lambda D,U1,U2,d. U1 d \land U2 d. 
+
+(* subset or (binary union) *)
+definition sor: \forall D. Subset D \to Subset D \to Subset D \def 
+   \lambda D,U1,U2,d. U1 d \lor U2 d. 
+
+(* subset less or equal (inclusion) *) 
+definition sle: \forall D. Subset D \to Subset D \to Prop \def 
+   \lambda D,U1,U2. \iforall d. U1 d \to U2 d. 
+
+(* subset overlap *) 
+definition sover: \forall D. Subset D \to Subset D \to Prop \def 
+   \lambda D,U1,U2. \iexists d. U1 d \land U2 d. 
+
+(* coercions **************************************************************)
+
+(*
+(* the class of the subsets of a domain (not an implicit coercion) *)
+definition class_of_subsets_of \def
+   \lambda D. mk_Class (Subset D) (true_f ?) (sle ?). 
+*)
+
+(* the domain built upon a subset (not an implicit coercion) *)
+definition domain_of_subset: \forall D. Subset D \to Domain \def
+   \lambda (D:Domain). \lambda U. 
+   mk_Domain (mk_Class D (sin D U) (cle1 D)).
+
+(* the full subset of a domain *)
+coercion stop.