(*******************)
intros. apply BV_to_nat_app. auto. unfold lenbv, lengthbv in |- *. apply len_length.
Qed. Hint Resolve BV_to_nat_app2.

Lemma BV_null_nat : forall n : nat, BV_to_nat (BV_null n) = 0.
unfold BV_null in |- *.