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_1: forall sz (m: word (S sz)) default,
pow2 sz <= # m < 2 * pow2 sz ->
@wmsb (S sz) m default = true.