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

Lemma xgcombine_l : forall (m: t A) (i : positive), get i (xcombine_l m) = f (get i m) None.