| nil2 : list2 A1 A2
| cons2: A1 → A2 → list2 A1 A2 → list2 A1 A2.
-interpretation "nil (list of pairs)" 'Nil2 = (nil2 ? ?). (**) (* 'Nil causes unification error in aacr_abst *)
+interpretation "nil (list of pairs)" 'Nil2 = (nil2 ? ?).
interpretation "cons (list of pairs)" 'Cons hd1 hd2 tl = (cons2 ? ? hd1 hd2 tl).
interpretation "append (list of pairs)"
'Append l1 l2 = (append2 ? ? l1 l2).
+
+let rec length2 (A1,A2:Type[0]) (l:list2 A1 A2) on l ≝ match l with
+[ nil2 ⇒ 0
+| cons2 _ _ l ⇒ length2 A1 A2 l + 1
+].
+
+interpretation "length (list of pairs)"
+ 'card l = (length2 ? ? l).