[ nil => l2
| (cons hd tl) => hd :: append A tl l2 ].
+definition tail := \lambda A:Set. \lambda l: list A.
+ match l with
+ [ nil => []
+ | (cons hd tl) => tl].
+
interpretation "append" 'append l1 l2 = (cic:/matita/list/append.con _ l1 l2).
theorem append_nil: \forall A:Set.\forall l:list A.l @ [] = l.