let rec tree_zip t1 t2 =
    match (t1,t2) with
    | (Empty, Empty) -> Empty
    | (_, Empty) -> raise Mismatch
    | (Empty, _) -> raise Mismatch
    | (Node (n1i, n1l), Node (n2i, n2l)) ->
    let f (a,b) = tree_zip a b in
        Node ((n1i, n2i, List.map f(zip n1l n2l)));;