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_map : forall (A B:Set) (f:A -> B) i l, Lget i (List.map f l) = match Lget i l with Some a => Some (f a) | None => None end.