-rec definition append2 (A1,A2:Type[0]) (l1,l2:list2 A1 A2) on l1 ≝ match l1 with
-[ nil2 ⇒ l2
-| cons2 a1 a2 tl ⇒ {a1, a2} @ append2 A1 A2 tl l2
-].
-
-interpretation "append (list of pairs)"
- 'Append l1 l2 = (append2 ? ? l1 l2).
-
-rec definition length2 (A1,A2:Type[0]) (l:list2 A1 A2) on l ≝ match l with