code>key</code> can be used to shorten the specification of
<a href="../reference/key_extraction.html#member"><code>member</code></a>,
<a href="../reference/key_extraction.html#const_mem_fun"><code>const_mem_fun</code></a>
(and <a href="../reference/key_extraction.html#cv_mem_fun">variants</a>),
<a href="../reference/key_extraction.html#mem_fun"><code>mem_fun</code></a>
(and <a href="../reference/key_extraction.html#volatile_mem_fun">variants</a>),
<a href="../reference/key_extraction.html#global_fun"><code