title: Filtering lists
body: <p>I want to filter repeated elements in my list
  for instance</p>
  
  <pre><code>foo = ['a','b','c','a','b','d','a','d']
  </code></pre>
  
  <p>I am only interested with:</p>
  
  <pre><code>['a','b','c','d']
  </code></pre>
  
  <p>What would be the efficient way to do achieve this ?
  Cheers</p>