maybe<typename std::size_t> minimum_idx_by_maybe(Compare comp,
        const Container& xs)
{
    if (is_empty(xs))
        return {};
    else
        return minimum_idx_by(comp, xs);
}
typename std::size_t minimum_idx(const Container& xs)
{
    return minimum_idx_by(is_less<typename Container::value_type>, xs);
}