hat the key extractor be read/write
when using the <code>modify_key</code> member function of key-based
indices. In all other situations, read-only extractors suffice.
The section on <a href="#advanced_key_extractors">advanced features
of Boost.MultiIndex key extractors</a> shows typical situations on which the predefined
key extractors are read/write.
</p>

<h2><a name="predefined_key_extractors">Predefined key extractors</a></h2>

<h3><a name="identity"><code>identity</code></a></h3>

<p>
The <a href="../