title: Nested list comprehensions in Julia
body: <p>In python I can do nested list comprehensions, for instance I can flatten the following array thus:</p>
  
  <pre><code>a = [[1,2,3],[4,5,6]]
  [i for arr in a for i in arr]
  </code></pre>
  
  <p>to get <code>[1,2,3,4,5,6]</code></p>
  
  <p>If I try this syntax in Julia I get:</p>
  
  <pre><code>julia&gt; a
  ([1,2,3],[4,5,6],[7,8,9])
  
  julia&gt; [i for arr in a for i in arr]
  ERROR: syntax: expected ]
  </code></pre>
  
  <p>Are nested list comprehensions in Julia possible?</p>