struct S
{
    template <typename F>
    int go(int a, int b, int c, int d, F f)
    {
        if (a != 0) return 0;
        if (a < 1) f(d, go(1, b, 1, d, f));
        f(d, go(1, 1, c, d, f));
        return d;
    }
};
S s;
int main()
{
    auto f = [](int &a, int b) { a = b; };
    s.go(0, 0, 0, 0, f);
    return 0;
}