Fixpoint nth_error_opt {A : Type} {m : nat} (v : Vector.t A m) (n : nat) : option A :=
  match n with
  | O =>
    match v with
    | Vector.nil _ => None
    | Vector.cons _ h _ _ => Some h
    end
  | S p =>
    match v with
    | Vector.nil _ => None
    | Vector.cons _ _ n1 t0 => nth_error_opt t0 p
    end
  end.

Lemma fin_of_lt_irrel: forall {m n : nat} (H1 H2 : Nat.lt n m), Fin.of_nat_lt H1 = Fin.of_nat_lt H2.