title: How to check if an element is in a nested list?
body: <p>How do I check if an element is in a nested list?</p>
  
  <p>I am trying to define a function <code>nested(x, ys)</code> that tests if a value <code>x</code> appears inside of a nested list of integers <code>ys</code>. The result has to have the value <code>True</code> of <code>False</code>. </p>