Fixpoint combine (m1: t A) (m2: t B) {struct m1} : t C :=
    match m1 with
    | Leaf => xcombine_r m2
    | Node l1 o1 r1 =>
        match m2 with
        | Leaf => xcombine_l m1
        | Node l2 o2 r2 => Node' (combine l1 l2) (f o1 o2) (combine r1 r2)
        end
    end.

Theorem gcombine: forall (m1: t A) (m2: t B) (i: positive), get i (combine m1 m2) = f (get i m1) (get i m2).