title: Why is back ticks for default of UIAlertActionStyle enum?
body: <p>I am a beginner to iOS (came from the Android background). I was looking at the documentation of UIViewController and noticed:    </p>
  
  <pre><code>@available(iOS 9.0, *)
  public enum UIPreviewActionStyle : Int {
      case `default`
      case selected
      case destructive
  } 
  </code></pre>
  
  <p>From Apple doc:   </p>
  
  <pre><code>case `default` = 0
  case cancel = 1
  case destructive = 2 
  </code></pre>
  
  <p><strong>I am confused about the back ticks for <code>'default'</code></strong>. Is it different values for the same enum or am I missing something about Swift enum?</p>