Definition semantic p := closed_under_proc (fun s t => s ≈ t) p.

(* Specifications: *)
Lemma unrecognisable_russell : ~ recognisable (fun s => closed s /\ ~ eva (s (tenc s))).
Lemma Reduction p f v :
proc v ->
(forall s, closed s -> p (f s) <-> ~ eva (s (tenc s))) ->
(forall s, v (tenc s) ≡ tenc (f s)) ->
~ recognisable p.