Fixpoint ltList (l1 l2 : list positive) :=
    match l1, l2 with
      | nil, nil => False
      | (hd1 :: tl1), (hd2 :: tl2) => if (Pos.ltb hd1 hd2) then True
                                      else if (Pos.ltb hd2 hd1) then False
                                           else (ltList tl1 tl2)
      | nil, _ => True
      | _, nil => False
    end.

Lemma sortOK : forall m l, length l = m -> length (sort l) = m.