Section plus.
Variable (a b c : nat-> nat) (ca cb cc : nat)
(Ha : is_cipher_of a ca)
(Hb : is_cipher_of b cb)
(Hc : is_cipher_of c cc).
Definition Code_plus := ca = cb + cc.

Lemma Code_plus_spec : Code_plus <-> forall i, i < l -> a i = b i + c i.