code> and <code>std::multimap</code> manage
elements of type <code>std::pair&lt;const Key,T&gt;</code>, where the first
member is the key. In either case, the process of obtaining the key from a
given element is implicitly fixed and cannot be customized by the user.
</p>

<p>
Fixed key extraction mechanisms like those performed by STL associative
containers do not scale well in the context of Boost.MultiIndex, where
several indices share their <code>value_type</code> definition but
might feature completely 