title: Remove elements from one Hashset where NOT in another Hashset?
body: <p>I am aware of <code>hashsetA.Except(hashsetB)</code> to remove elements from <code>hashsetA</code> that exist in <code>hashsetB</code>. However, I want to remove elements from <code>hashsetA</code> that <strong>don't</strong> exist in <code>hashsetB</code>.</p>
  
  <p>Currently I just copy <code>hashsetA</code> to a new <code>Hashset</code> then use <code>ExceptWith()</code> twice:</p>
  
  <pre><code>hashsetC = new HashSet&lt;var&gt;(hashsetA);
  hashsetC.ExceptWith(hashsetB);
  hashsetA.ExceptWith(hashsetC);
  </code></pre>
  
  <p>The performance of this is plenty good enough for my purposes, but I was wondering if there's a built in method to make this faster/more concise?
  Or am I missing an obvious way to select from the sets? </p>