Re: [hub] on the specialized iteration over elements

It seems to me that 'visit' is useful and legal only in combination
with 'operator++',
'operator--' and the likes of 'std::next', 'std::prev',
'std::advance', etc. For example:

    int main() {
        boost::container::hub<int> h {1, 3, 5, 7, 9};
        auto first = h.begin();
        auto middle = std::next(first, h.size() / 2);
        auto last = h.end();
        h.visit(first, middle, [](int i){
            std::print("{} ", i);
        });
        std::print(" | ");
        h.visit(middle, last, [](int i){
            std::print("{} ", i);
        });
    }

(https://godbolt.org/z/1e5E413r8)

Important use cases are divide-and-conquer algorithms and parallelization.

>From here, two things.

If 'visit' is faster than just "ranging over", it probably (?) means
that we need
'hub::advance' etc as well.

But then an integer displacement is always involved, be it 1 or -1.
Doesn't it suggest
that 'visit' should take offsets, not iterators? That would take care
of Andrzej's
concern. And there is no need for public 'advance' etc members.
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