Fixpoint all_but (i : nat) {A} (l : list A) : list A :=
  match i with
    O => tl l
  | S i =>
    match l with
      nil => nil
    | x :: l => x :: all_but i l
    end
  end.

Lemma all_but_app {A} i (l l' : list A) : lt i (List.length l) -> all_but i (l ++ l') = all_but i l ++ l'.