Stability and insertion with hint in ordered associative containers with equivalent keys

Boost.Intrusive ordered associative containers with equivalent keys offer stability guarantees, following C++ standard library's defect #233 resolution, explained in document Comments on LWG issue 233: Insertion hints in associative containers. This means that:

A Insert without hint member function always insert at the upper bound of an equal range.
A Insert with hint member function inserts the new value before the hint if hint's and new node's keys are equivalent.
Implements Andrew Koenig as close as possible to hint proposal. A new element is always be inserted as close to the hint as possible. So, for example, if there is a subsequence of equivalent values, a.begin() as the hint means that the new element should be inserted before the subsequence even if a.begin() is far away. This allows code to always append (or prepend) an equal range with something as simple as: m.insert(m.end(), new_node); or m.insert(m.begin(), new_node);

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