--- /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 "ground/notation/relations/ringeq_3.ma".
+include "ground/lib/list.ma".
+
+(* EXTENSIONAL EQUIVALENCE OF LISTS *****************************************)
+
+rec definition eq_list A (l1,l2:list A) on l1 ≝
+match l1 with
+[ nil ⇒
+ match l2 with
+ [ nil ⇒ ⊤
+ | cons _ _ ⇒ ⊥
+ ]
+| cons a1 l1 ⇒
+ match l2 with
+ [ nil ⇒ ⊥
+ | cons a2 l2 ⇒ a1 = a2 ∧ eq_list A l1 l2
+ ]
+].
+
+interpretation "extensional equivalence (list)"
+ 'RingEq A l1 l2 = (eq_list A l1 l2).
+
+(* Basic properties *********************************************************)
+
+lemma eq_list_refl (A): reflexive … (eq_list A).
+#A #l elim l -l /2 width=1 by conj/
+qed.
+
+(* Main properties **********************************************************)
+
+theorem eq_eq_list (A,l1,l2): l1 = l2 → l1 ≗{A} l2.
+// qed.
+
+(* Main inversion propertiess ***********************************************)
+
+theorem eq_list_inv_eq (A,l1,l2): l1 ≗{A} l2 → l1 = l2.
+#A #l1 elim l1 -l1 [| #a1 #l1 #IH ] *
+[ //
+| #a2 #l2 #H elim H
+| #H elim H
+| #a2 #l2 * #Ha #Hl /3 width=1 by eq_f2/
+]
+qed-.