ification of
<code>multi_index_container</code>s as their elements are treated as constant,
but the section on <a href="#advanced_key_extractors">advanced features
of Boost.MultiIndex key extractors</a> describes valid use cases of
key extraction based on such functions with a non-constant reference argument.
</p>

<p><a href="../examples.html#example2">Example 2</a> in the examples section
uses <code>gobal_fun</code>.
<p>

<h2><a name="user_defined_key_extractors">User-defined key extractors</a></h2>

<p>
