title: How to mutate Swift Dictionary of Dictionary
body: <p>I have a Swift Dictionary that contains Dictionary, and I wanted to use stored property to access a key-value:</p>
  
  <pre><code>  var json = [NSObject:AnyObject]()
    var title: String {
      get {
        return (self.json["category"]! as [NSObject: AnyObject])["title"] as String
      }
      set {
  //      var dict = (self.json["category"]! as [NSObject: AnyObject])
  //      dict["title"] = newValue
  
        (self.json["category"]! as [NSObject: AnyObject])["title"] = newValue
      }
    }
  </code></pre>
  
  <p>The last line throws an compilation error: Operand of postfix '!' should have optional type; type is '(NSObject, AnyObject)'</p>
  
  <p>Apparently based on the compilation error, the compiler think self.json["category"] is type of (NSObject, AnyObject) that cannot be be converted to Dictionary!</p>
  
  <p>I thought this should be a common usage? How would you do normally to access Dictionary of Dictionary?</p>