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.

Lemma WS_eq_rect : forall b n (w: word n) n' H H',
eq_rect _ word (@WS b n w) _ H = @WS b n' (eq_rect _ word w _ H').