title: Convert NSNumber to NSDecimalNumber
body: <p>I realize that NSDecimalNumber inherits from NSNumber.</p>
  
  <p>However, I am struggling to figure out how I can init an NSDecimalNumber from an NSNumber or how to convert an NSNumber to an NSDecimalNumber.</p>
  
  <p>I have tried</p>
  
  <pre><code>NSDecimalNumber* d = [aNumber copy];
  </code></pre>
  
  <p>where <code>aNumber</code> is an NSNumber object to no avail.</p>
  
  <p>Am I missing something?</p>
  
  <p>Thanks.</p>