title: Add field to enum in Swift
body: <p>I'm dealing with enum and subclassing in Swift.
  Each child bring its own new properties which have to be stored in an Enum. This enum is declared with some values in the mother class. I'd like to add some value to this enum. I can't find out how to do it, I tried this without result :</p>
  
  <pre><code>extension MotherClass {
      enum Enumeration {
          case NewProperty
      }
  }
  </code></pre>