class Node
{
    int data; // or w/e

    Node * left;
    Node * right;

    template <typname Action>
    void SerialTraverse(Action action)
    {
        action(data); // Pre-order traversal

        if (left) left->SerialTraverse(action);
          // Traverse the left
        if (right) right->SerialTraverse(action);
          // Traverse the right
    }
}

namespace library
{
    template<typename Task, typename Class, typename Arg>
    std::future<void> enqueue_task(Task task, Class * obj, Arg arg);
    // on some other thread, call "obj->task(arg);"
    // returns a handle upon which we can wait
}

template <typname Action>
    void ParallelTraverse(Action action)
    {
        action(data);
        std::future<void> fut;
        if (left) fut = library::enqueue_task(ParallelTraverse, left, action);
          // start traversing left on another thread

        if (right) right->ParallelTraverse(action);
          // traverse right on this thread, in parallel to traversing left

        if (fut.valid()) fut.wait();
          // wait for the left traversal to end
    }