tion signature depending on the result of some condition,
we can decide whether or not an overload is considered during overload
resolution.  The technique is formalized in
the <a class="reference external" href="../../../utility/enable_if.html"><tt class="docutils literal">enable_if</tt></a> utility.  Most recent compilers support SFINAE;
on compilers that don't support it, the Boost config library
will <tt class="docutils literal">#define</tt> the symbol <tt class="docutils literal">BOOST_NO_SFINAE</tt>.
