Definition creative P := recognisable P /\ productive (complement P).

(* Specifications: *)
Lemma productive_K : productive (fun s : term => ~ eva (s (tenc s))).
Lemma productive_red P Q : productive P -> P ⪯ Q -> productive Q.
Lemma extend g :
computable_fun g -> exists k, computable_fun k /\ forall s t, pi (k s) t <-> pi s t \/ t = g s.