(* Replacing BVxor *)
Definition BLxor (xs : Blist) (ys : Blist) :=
  map (fun p => xorb (fst p) (snd p)) (combine xs ys).

Lemma BLxor_length: forall n l1 l2 (len1: length l1 = n) (len2: length l2 = n), length (BLxor l1 l2) = n.