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

Lemma sublist_0_cons' : forall {A} i j (x : A) l, i <= 0 -> j > i -> sublist i j (x :: l) = x :: sublist i (j - 1) l.