intro.
rewrite (shatter_word a).
replace (wtl a) with WO by auto.
destruct (whd a).
right; apply eq_refl.
left; apply eq_refl.
Defined.

Lemma wmsb_default:
forall sz (w: word sz) b1 b2,
sz <> 0 -> wmsb w b1 = wmsb w b2.