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_eq_rect2 : forall sz n n'
(H: n = n') H'
(a: word sz) (b: word n),
combine a b =
eq_rect _ word (combine a (eq_rect _ word b _ H)) _ H'.