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diff --git a/matita/matita/library/datatypes/compare.ma b/matita/matita/library/datatypes/compare.ma
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+(**************************************************************************)
+(*       ___	                                                            *)
+(*      ||M||                                                             *)
+(*      ||A||       A project by Andrea Asperti                           *)
+(*      ||T||                                                             *)
+(*      ||I||       Developers:                                           *)
+(*      ||T||       A.Asperti, C.Sacerdoti Coen,                          *)
+(*      ||A||       E.Tassi, S.Zacchiroli                                 *)
+(*      \   /                                                             *)
+(*       \ /        This file is distributed under the terms of the       *)
+(*        v         GNU Lesser General Public License Version 2.1         *)
+(*                                                                        *)
+(**************************************************************************)
+
+inductive compare :Set \def
+| LT : compare
+| EQ : compare
+| GT : compare.
+
+definition compare_invert: compare \to compare \def
+  \lambda c.
+    match c with
+      [ LT \Rightarrow GT
+      | EQ \Rightarrow EQ
+      | GT \Rightarrow LT ].