title: QSet in QMap or QHash
body: <p>I have <code>QMap</code>  and I want to make <code>QSet</code> the key of it, I couldn't do that because <code>QSet</code> is not comparable.
  for example:</p>
  
  <pre><code>QSet&lt;int&gt; intSet;
  QMap&lt;QSet&lt;int&gt;, char&gt; charSet;
  
  intSet.insert(1);
  intSet.insert(2);
  intSet.insert(3);
  
  charSet.insert(intSet, '6');
  </code></pre>
  
  <p>Is there any way to make it work? and if I inherit from <code>QSet</code> and define <code>operator &lt;</code> how should I implement it? i.e: What should be the logic of the comparison?</p>
  
  <p>Note: I care too much about performance</p>