repeat match goal with
| [ |- context[wordToNat (natToWord ?sz ?w)] ] =>
let Heq := fresh "Heq" in
destruct (wordToNat_natToWord sz w) as [? [Heq ?]]; rewrite Heq
end.
rewrite mult_minus_distr_r.
match goal with
| [ |- natToWord _ (_ - ?x0' * _ * _) = natToWord _ (_ - ?x1' * _ + (_ - ?x2' * _)) ]
=> rename x0' into x0, x1' into x1, x2' into x2 (* force the names to not be autogenerated *)
end.
rewrite <- (mult_assoc x0).
rewrite (mult_comm (pow2 sz)).
rewrite (mult_assoc x0).
replace (wordToNat x * wordToNat z - x1 * pow2 sz +
(wordToNat y * wordToNat z - x2 * pow2 sz))
with (wordToNat x * wordToNat z + wordToNat y * wordToNat z - x1 * pow2 sz - x2 * pow2 sz).
repeat rewrite drop_sub; auto with arith.
rewrite (mult_comm x0).
rewrite (mult_comm (wordToNat x + wordToNat y)).
rewrite <- (mult_assoc (wordToNat z)).
auto with arith.
generalize dependent (wordToNat x * wordToNat z).
generalize dependent (wordToNat y * wordToNat z).
intros.
omega.
Qed.

(* Specifications: *)
Theorem wminus_def : forall sz (x y : word sz), x ^- y = x ^+ ^~ y.
reflexivity.