Fixpoint compare_list {A: Type} (f: A -> A -> comparison) (xl yl : list A)
  : comparison :=
  match xl , yl with
  | x :: xl', y :: yl' => match f x y with
                          | Eq => compare_list f xl' yl'
                          | c => c
                          end
  | nil , _ :: _ => Lt
  | _ :: _ , nil => Gt
  | nil, nil => Eq
  end.

Lemma NoDup_rsort_uniq: forall {A} (cmp: A -> A -> comparison) (cspec: CompSpec' cmp), forall l, NoDup (rsort_uniq cmp l).