(** Particular case : the second bit of an addition *)

(* Specifications: *)
Lemma add_bit1 : forall a b,
(a+b).[1] = xor3 a.[1] b.[1] (a.[0] && b.[0]).
Lemma add_bit1 : forall a b,
(a+b).[1] = xor3 a.[1] b.[1] (a.[0] && b.[0]).