title: Get 'undefined' if 'window.undefined' is overwritten
body: <p>It appears that <code>window.undefined</code> is writable, i.e. it can be set to something else than its default value (which is, unsurprisingly, <code>undefined</code>).</p>
  
  <p>The point is however that whenever I refer to <code>undefined</code>, it refers to <code>window.undefined</code> (as <code>window</code> can be removed in cases like this).</p>
  
  <p>So how do I actually get access to an <code>undefined</code> 'instance', so to say? How would I be able to set another variable to <code>undefined</code>, if <code>window.undefined</code> has been changed?</p>
  
  <p>If I code:</p>
  
  <pre><code>window.undefined = 'foo'; // This code might have been executed by someone/something
  var blah = undefined; // blah is not undefined, but equals to 'foo' instead...
  </code></pre>
  
  <p>How could I possibly solve this?</p>