Definition mul' (x y: Int.int) : int := repr (Int.unsigned x * Int.unsigned y).

Lemma decompose_mul_2:
forall xh xl yh yl,
mul (ofwords xh xl) (ofwords yh yl) =
ofwords (Int.add (Int.add (Int.mulhu xl yl) (Int.mul xl yh)) (Int.mul xh yl))
(Int.mul xl yl).