title: Array from set: why does NSSet use allObjects, while NSOrderedSet uses array?
body: <p>In Foundation, if I want to convert a set to an <code>NSArray</code>, I can use:</p>
  
  <ul>
  <li><code>-[NSSet allObjects]</code></li>
  <li><code>-[NSOrderedSet array]</code></li>
  </ul>
  
  <p>Why are these different?</p>