intuition.
unfold bvToNat.
assert (N.to_nat N0 = O).
simpl.
trivial.
rewrite <- H.
f_equal.
eapply Bv2N_zero.
Qed.
Definition natToBv(k : nat)(v : nat) : Bvector k :=
N2Bv_gen k (N.of_nat v).

Lemma Bv2N_app_false : forall n1 n2 (v1 : Bvector n1),
Bv2N (n1 + n2) (Vector.append v1 (Bvect_false n2)) = Bv2N n1 v1.