e>s
of elements of <code>T</code> cannot be sorted by <code>T::g</code> or <code>T::gg</code>, because
elements contained within a <code>multi_index_container</code> are treated as constant.
</p>

<p>
The column "read/write?" shows which combinations yield
<a href="#read_write_key_extractors">read/write key extractors</a>.
</p>

<p>
Some care has to be taken to preserve <code>const</code>-correctness in the
specification of <code>member</code> key extractors: in some sense, the <code>const</code>
qualifier 