template<class F>
tree find(const tree& where, F&& predicate)
{
    tree ret;
    find_aux(where, std::forward<F>(predicate), ret);
    return ret;
}

template<class F>
void find_aux(const tree& where, F&& predicate, tree& accum)
{
    if (predicate(where.value)) accum.add(where.value);

    find_aux(where.left,  std::forward<F>(predicate), accum);
    find_aux(where.right, std::forward<F>(predicate), accum);
}