title: When wouldn't NSObject be stated as the parent class?
body: <p>For example:</p>
  
  <pre><code>@interface Fraction: NSObject {
  ...
  </code></pre>
  
  <p>When wouldn't <code>NSObject</code> be used and is <code>NSObject</code> the ultimate parent class for all other classes?</p>
  
  <p>Please feel free to correct me on any wrong terminology used.</p>