intuition.
rewrite <- BVxor_id_l at 1.
rewrite <- (BVxor_id_l v2).
rewrite <- (BVxor_same_id v2) at 1.
rewrite BVxor_assoc.
rewrite BVxor_comm.
f_equal; intuition.
rewrite BVxor_comm.
trivial.
Qed.
Definition lognat(n : nat) : nat :=
N.size_nat (N.of_nat n).
Definition bvToNat(k : nat)(v : Bvector k) :=
N.to_nat (Bv2N k v).

Lemma Bv2N_zero : forall (n : nat),
Bv2N n (Bvect_false n) = N0.