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<p>Sort a group according to the given comparison function. </p>
<p>The comparison function object must return <code>true</code> if the first element is <em>less</em> than the second one, <code>false</code> otherwise. The signature of the comparison function should be equivalent to one of the following:</p>
<div class="fragment"><div class="line">bool(std::tuple&lt;Type &amp;...&gt;, std::tuple&lt;Type &amp;...&gt;);</div>
<div class="line">bool(<span class="keyword">co