title: namedtuple._replace() doesn't work as described in the documentation
body: <p>I was having trouble implementing <code>namedtuple._replace()</code>, so I copied the code right off of the documentation:</p>
  
  <pre><code>Point = namedtuple('Point', 'x,y')
  
  p = Point(x=11, y=22)
  
  p._replace(x=33)
  
  print p
  </code></pre>
  
  <p>and I got:</p>
  
  <pre><code>Point(x=11, y=22)
  </code></pre>
  
  <p>instead of:</p>
  
  <pre><code>Point(x=33, y=22)
  </code></pre>
  
  <p>as is shown in the doc.</p>
  
  <p>I'm using Python 2.6 on Windows 7</p>
  
  <p>What's going on?</p>