title: Python's "in" set operator
body: <p>I'm a little confused about the python <code>in</code> operator for sets.</p>
  
  <p>If I have a set <code>s</code> and some instance <code>b</code>, is it true that <code>b in s</code> means "<em>is there some element <code>x</code> in <code>s</code> such that <code>b == x</code> is <code>true</code></em>"?</p>