intro.
rewrite (shatter_word a).
replace (wtl a) with WO by auto.
destruct (whd a).
right; apply eq_refl.
left; apply eq_refl.
Defined.

Theorem wbit_and_not: forall sz sz' (n : word sz'), (wordToNat n < sz)%nat
-> (wbit sz n) ^& wnot (wbit sz n) = wzero sz.