(* Section 4.1, equation 19 *)
Definition approx_eq (n : nat) (tau1 tau2 : predicate) : Prop :=
  approx n tau1 = approx n tau2.

(* Specifications: *)
Lemma approx_eq_downward : forall n m p q, m <= n -> approx_eq n p q -> approx_eq m p q.