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_natToWord: forall sz n default,
(2 * n < pow2 (S sz))%nat ->
@wmsb (S sz) (natToWord (S sz) n) default = false.