static struct {
        template<typename R>
        R operator() (const Tree<T>& tree, std::function<R(T, R)> f, R initial) {
            return _fold([=](NodePtr node, std::function<R(R)> l, std::function<R(R)> r) {
                return [=](R x) -> R { return r(f(node->data(), l(x))); };
            }, std::function<R(R)>([](R x) -> R { return x; }), tree._root)(initial);
        }
    } inorder;

    template<typename Order>
    void print(Order order) const {
        order(*this,
              std::function<int(T data, int)>(
                  [](T data, int) -> int { std::cout << data << " "; return 0; }),
              0);
    }