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_nat_cast:
forall sz1 (w: word sz1) sz2 (Hsz: sz1 = sz2) b,
wmsb w b = wmsb (nat_cast word Hsz w) b.