Fact is_cipher_of_u : l+1 < q -> is_cipher_of (fun _ => 1) (∑ l (fun i => power (power (S i) 2) r)).
(* Fact is_cipher_of_u1 : l+1 < q -> is_cipher_of (fun _ => 1) (∑ l (fun i => power (power (S (S i)) 2) r)). *)
Definition the_cipher f : l+1 < q -> (forall i, i < l -> f i < power q 2) -> { c | is_cipher_of f c }.
Definition Code a := exists f, is_cipher_of f a.

Lemma Code_dio a : Code a <-> l = 0 /\ 1 < q /\ a = 0
\/ 0 < l /\ l+1 < q /\ exists p u u1, p+1 = power q 2 /\ seqs_of_ones u u1 /\ a ≲ p*u.