(* listRepeat is used to show some interesting properties of rel_map and pred_count. *)
Fixpoint listRepeat(A : Type)(a : A) n :=
  match n with
    | 0 => nil
    | S n' => a :: (listRepeat a n')
  end.

Lemma pred_count_eq_all : forall (A : Type)(P : A -> Prop)(ls : list A) n, (forall a, In a ls -> P a) -> n = length ls -> pred_count P ls n.