induction n1 as [|n1 IHn1]; simpl; intros n2 n3 Heq w; intuition.
rewrite (UIP_dec eq_nat_dec Heq (refl_equal _)); reflexivity.
rewrite (IHn1 _ _ (plus_assoc _ _ _)).
f_equal.
apply wtl_match.
Qed.

Theorem combine_n_0 : forall sz1 (w : word sz1) (v : word 0),
combine w v = eq_rect _ word w _ (plus_n_O sz1).