st always you will want to use a <code>const</code> member function,
since elements in a <code>multi_index_container</code> are treated as constant, much
as elements of an <code>std::set</code>. However, a
<a href="../reference/key_extraction.html#mem_fun"><code>mem_fun</code></a>
counterpart is provided for use with non-constant member functions, whose
applicability is discussed on the paragraph on
<a href="#advanced_key_extractors">advanced features
of Boost.MultiIndex key extractors</a>.
</p>

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