title: How to find the index of the nth time an item appears in a list?
body: <p>Given:</p>
  
  <pre><code>x = ['w', 'e', 's', 's', 's', 'z','z', 's']
  </code></pre>
  
  <p>Each occurrence of <code>s</code> appears at the following indices:</p>
  
  <p>1st: 2<br>
  2nd: 3<br>
  3rd: 4<br>
  4th: 7  </p>
  
  <p>If I do <code>x.index('s')</code> I will get the 1st index.</p>
  
  <p>How do I get the index of the 4th <code>s</code>?</p>