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_existT: (* Note: not axiom free *)
forall sz1 (w1: word sz1) sz2 (w2: word sz2),
existT word _ w1 = existT word _ w2 ->
forall b, wmsb w1 b = wmsb w2 b.