Hint Unfold Lambda : cbv.

(* Specifications: *)
Lemma Lambda_correct s : Lambda (tenc s) ▷ T /\ lam s \/
Lambda (tenc s) ▷ F /\ ~ lam s.
Lemma decidable_lam : decidable lam.
Lemma decidable_proc : decidable proc.
Notation "s '≈' t" := (forall u v, t (tenc u) ▷ v <-> s (tenc u) ▷ v) (at level 50).