title: python set comprehension made out of different lists
body: <pre><code>expected = {
      'l1': ['abc', 'def', 'ghi', 'jkl'],
      'l2': ['abc', 'ghi', 'jkl', 'mno']
  }
  </code></pre>
  
  <p>I would like to get <code>set(['abc', 'def', 'ghi', 'jkl', 'mno'])</code> using python 2.6+ (so <code>{x for x in ...}</code> is not what I want).</p>
  
  <p>I've tried</p>
  
  <pre><code>all_files = set(files for files in expected.values())
  </code></pre>
  
  <p>but it throws:</p>
  
  <pre><code>TypeError: unhashable type: 'list'
  </code></pre>