title: What is the purpose of SelectMany(x => x)?
body: <p>I understand the use of lambda functions as a filter such as <code>(x =&gt; x.Count() == 1)</code>, but what is the purpose of the <code>(x =&gt; x)</code>? When I take it out, the code doesn't compile, and every example of lambda functions I can find seems to use it to filter in one line instead of multiple lines without the lambda.</p>
  
  <pre><code>List&lt;Tuple&lt;int, int&gt;&gt; regVals = ReadRegValCollection.SelectMany(x =&gt; x).ToList();
  </code></pre>
  
  <p>The purpose of this gem is to flatten a List of Lists into a List</p>