title: Flatten doubly nested lists
body: <p>How to convert this:</p>
  
  <pre><code>[[[1,2,3], ['a','b','c']], [[4,5], ['d','e']], [[6,7,8,9], ['f','g','h','i']]]
  </code></pre>
  
  <p>to this:</p>
  
  <pre><code>[[1,2,3,4,5,6,7,8,9], ['a','b','c','d','e','f','g','h','i']]
  </code></pre>
  
  <p>Knowing python, there must be some way using zip and list comprehensions.</p>