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_0: forall sz (m: word (S sz)) default,
(# m < pow2 sz)%nat ->
@wmsb (S sz) m default = false.