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 wordnat_minus1_eq : forall sz n (w : word sz),
(n > 0)%nat
-> n = wordToNat w
-> n - 1 = wordToNat (w ^- $1).