Fixpoint xcombine_r (m : t B) {struct m} : t C :=
      match m with
      | Leaf => Leaf
      | Node l o r => Node' (xcombine_r l) (f None o) (xcombine_r r)
      end.

Lemma xgcombine_r : forall (m: t B) (i : positive), get i (xcombine_r m) = f None (get i m).