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

Lemma length_al :
forall t : nat, lengthbv (al t di_init cx_init al_init mem_init) = d_size.