rewrite wplus_alt.
unfold wplusN, wordBinN.
destruct (wordToNat_natToWord (S sz) n); intuition.
destruct (wordToNat_natToWord (S sz) m); intuition.
do 2 match goal with H : _ |- _ => rewrite H; clear H end.
match goal with
| [ |- context[?n - ?x * pow2 (S ?sz) + (?m - ?x0 * pow2 (S ?sz))] ]
=> replace (n - x * pow2 (S sz) + (m - x0 * pow2 (S sz))) with (n + m - x * pow2 (S sz) - x0 * pow2 (S sz))
by omega
end.
repeat rewrite drop_sub; auto; omega.
Qed.

Lemma natToWord_S : forall sz n, natToWord sz (S n) = natToWord _ 1 ^+ natToWord _ n.
intros sz n; change (S n) with (1 + n); apply natToWord_plus.