title: How to unzip an iterator?
body: <p>Given a list of pairs <code>xys</code>, the Python idiom to unzip it into two lists is:</p>
  
  <pre><code>xs, ys = zip(*xys)
  </code></pre>
  
  <p>If <code>xys</code> is an iterator, how can I unzip it into two iterators, without storing everything in memory?</p>