anchor.diff:  module Id =
     let mutable private id = 0L
   
  -  /// Returns a new unique id.
  +  /// Returns the next unique id.
     let next () = Interlocked.Increment &id
   
   [<STAThread; EntryPoint>]
  
positive.code:           if item.IsCompleted then FontStyles.Italic else FontStyles.Normal
  
          // This updates the underlying item in response to a click on the
          // completed check box.
          Stream.ofObservableOnMain control.IsCompleted.Click
          |> Stream.subscribeJob (fun _ -> onMain {
             let isCompleted = control.IsCompleted.IsChecked.Value
  
negative.code: // some builtin WPF controls and overuses choice streams to show some more
  // choice stream usage patterns.  WPF is a fundamentally imperative GUI
  // framework and this example does not try to rewrite WPF as a more declarative
  // framework.  Some of the simple techniques used in this example, such as using
  // an immutable map as the model and removing and adding all controls to a panel
  // when the model changes, do not necessarily scale.  Would have used XAML type
  // provider, but it currently does not mix with PCL.
  //
  