title: Creating unordered_set of unordered_set
body: <p>I want to create a container that will store unique sets of integers inside.</p>
  
  <p>I want to create something similar to</p>
  
  <pre><code>std::unordered_set&lt;std::unordered_set&lt;unsigned int&gt;&gt;
  </code></pre>
  
  <p>But g++ does not let me do that and says:</p>
  
  <pre><code>invalid use of incomplete type 'struct std::hash&lt;std::unordered_set&lt;unsigned int&gt; &gt;'
  </code></pre>
  
  <p>What I want to achieve is to have unique sets of unsigned ints.</p>
  
  <p>How can I do that?</p>