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diff --git a/matita/matita/contribs/lambdadelta/basic_2/i_static/rexs_length.ma b/matita/matita/contribs/lambdadelta/basic_2/i_static/rexs_length.ma
new file mode 100644
index 000000000..9b1e1263b
--- /dev/null
+++ b/matita/matita/contribs/lambdadelta/basic_2/i_static/rexs_length.ma
@@ -0,0 +1,29 @@
+(**************************************************************************)
+(*       ___                                                              *)
+(*      ||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                  *)
+(*                                                                        *)
+(**************************************************************************)
+
+include "basic_2/static/rex_length.ma".
+include "basic_2/i_static/rexs.ma".
+
+(* ITERATED EXTENSION ON REFERRED ENTRIES OF A CONTEXT-SENSITIVE REALTION ***)
+
+(* Forward lemmas with length for local environments ************************)
+
+(* Basic_2A1: uses: TC_lpx_sn_fwd_length *)
+lemma rexs_fwd_length: ∀R,L1,L2,T. L1 ⪤*[R, T] L2 → |L1| = |L2|.
+#R #L1 #L2 #T #H elim H -L2
+[ #L2 #HL12 >(rex_fwd_length … HL12) -HL12 //
+| #L #L2 #_ #HL2 #IHL1
+  >IHL1 -L1 >(rex_fwd_length … HL2) -HL2 //
+]
+qed-.