Fixpoint Lget (A:Set) (n:nat) (l:list A) {struct l}:option A :=
match l with nil => None
| x::q =>
match n with O => Some x
| S m => Lget A m q
end end .

Lemma Lget_app_Some : forall (A:Set) l delta i (a: A), Lget i l = Some a -> Lget i (l ++ delta) = Some a.