add example of indirect case (don't know why I didn't commit this long ago)

Author: mattneub

bk1ch04p142valueTypesAndReferenceTypes/bk1ch04p142valueTypesAndReferenceTypes/ViewController.swift

@@ -44,8 +44,14 @@ struct RecursiveDog { // compile error
 
 */
 
+// but this is now legal:
 
-
+enum Node {
+    case None(Int)
+    indirect case Left(Int, Node)
+    indirect case Right(Int, Node)
+    indirect case Both(Int, Node, Node)
+}
 
 class ViewController: UIViewController {