title: RemoveObjectAtIndex doesn't work
body: <p>I want to delete a object from my Array but it doesn't work!</p>
  
  <p>Thats my code:</p>
  
  <pre><code>[array removeObjectAtIndex:2];
  </code></pre>
  
  <p>It shows me:</p>
  
  <blockquote>
    <p>No visible @Interface for "NSArray" declares the selector "removeObjectAtIndex:"</p>
  </blockquote>
  
  <p>I work in Xcode 6.3.1 with Objective-C.</p>
  
  <p>What can I do?</p>