--- /dev/null
+(**************************************************************************)
+(* ___ *)
+(* ||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 "static_2/syntax/tueq.ma".
+
+(* TAIL SORT-IRRELEVANT EQUIVALENCE ON TERMS ********************************)
+
+(* Main properties **********************************************************)
+
+theorem tueq_trans: Transitive … tueq.
+#T1 #T #H elim H -T1 -T
+[ #s1 #s #X #H
+ elim (tueq_inv_sort1 … H) -s /2 width=1 by tueq_sort/
+| #i1 #i #H //
+| #l1 #l #H //
+| #p #I #V #T1 #T #_ #IHT #X #H
+ elim (tueq_inv_bind1 … H) -H /3 width=1 by tueq_bind/
+| #V #T1 #T #_ #IHT #X #H
+ elim (tueq_inv_appl1 … H) -H /3 width=1 by tueq_appl/
+| #V1 #V #T1 #T #_ #_ #IHV #IHT #X #H
+ elim (tueq_inv_cast1 … H) -H /3 width=1 by tueq_cast/
+]
+qed-.
+
+theorem tueq_canc_sn: left_cancellable … tueq.
+/3 width=3 by tueq_trans, tueq_sym/ qed-.
+
+theorem tueq_canc_dx: right_cancellable … tueq.
+/3 width=3 by tueq_trans, tueq_sym/ qed-.
+
+theorem tueq_repl: ∀T1,T2. T1 ≅ T2 →
+ ∀U1. T1 ≅ U1 → ∀U2. T2 ≅ U2 → U1 ≅ U2.
+/3 width=3 by tueq_canc_sn, tueq_trans/ qed-.