Fixpoint extend {A} (l : list A) ls :=
  match l, ls with
  | x :: xs, y :: ys => (x :: y) :: extend xs ys
  | _, _ => ls
  end.

Lemma Znth_extend_in : forall {A}{d: Inhabitant A} (l : list A) ls i, 0 <= i < Zlength l -> Zlength l <= Zlength ls -> Znth i (extend l ls) = Znth i l :: Znth i ls.