Fixpoint extendr {A} (l : list A) ls :=
match l, ls with
| x :: xs, y :: ys => (y ++ [x]) :: extendr 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.