li>
  <li><a href="#advanced_key_extractors">Advanced features of Boost.MultiIndex key
    extractors</a></li>
</ul>

<h2><a name="intro">Introduction</a></h2>

<p>
STL associative containers have a notion of key, albeit in a somewhat incipient
form. So, the keys of such containers are identified by a nested type
<code>key_type</code>; for <code>std::set</code>s and <code>std::multiset</code>s,
<code>key_type</code> coincides with <code>value_type</code>, i.e. the key is the
element itself. <code>std::map</