induction sz1; shatterer.
Qed.
Hint Rewrite combine_split.
Hint Rewrite split1_combine.
Hint Rewrite split2_combine.

Theorem combine_assoc : forall n1 (w1 : word n1) n2 n3 (w2 : word n2) (w3 : word n3) Heq,
combine (combine w1 w2) w3
= match Heq in _ = N return word N with
| refl_equal => combine w1 (combine w2 w3)
end.