title: Multiple repeated `in` keyword
body: <p>By mistake I have used more than one <code>in</code> keyword in an expression, but the code still works.</p>
  
  <p>What is the meaning of:</p>
  
  <pre><code>"a" in "bar" in "foo"   # in ... ?
  </code></pre>
  
  <p>naively I thought that this was equivalent to <code>("a" in "bar") in "foo"</code> or <code>"a" in ("bar" in "foo")</code> but it is not the case since both are not valid. I get the same behaviour in python2 or 3.</p>