title: NSMutableOrderedSet and NSNull behavior?
body: <p>I don't understand why I can't add many [NSNull null] to a NSMutableOrderedSet:</p>
  
  <pre><code>NSMutableOrderedSet *set = [[NSMutableOrderedSet alloc]init];
  for(int i = 0; i &lt; 10; i++)
      {
          [set addObject: [NSNull null]];
      }
  
  NSLog("SET COUNT : %d", set.count);
  </code></pre>
  
  <p>This Output : </p>
  
  <pre><code>SET COUNT : 1
  </code></pre>
  
  <p>And if I try it with a NSMutableArray it works just fine, I've read NSNull documentation (which is really short but didn't help) but it says:</p>
  
  <pre><code>"The NSNull class defines a singleton object used to represent null values in collection objects"
  </code></pre>
  
  <p>and NSMutableOrderedSet documentation and it says it's an </p>
  
  <pre><code>"[...] ordered collection of distinct objects"
  </code></pre>
  
  <p>So if it's a collection why isn't it working ?</p>
  
  <p>Thank you very much</p>