simple induction t. simpl in |- *. exact di_initsize.
intros.
rewrite di_t. elim (IsNull (cx n di_init cx_init al_init mem_init)). exact H.
rewrite length_BV_increment. exact H.
Qed.

Lemma length_cx :
forall t : nat, lengthbv (cx t di_init cx_init al_init mem_init) = a_size.