anchor.diff:  
   let mkField<'T> propertyExpr (initialValue: 'T) (obs: NotifyBase) = obs.MakeField (propertyExpr, initialValue)
   
  -let mkCommand<'T> canExecuteHandler executeHandler (notifyOnFieldUpdate: FieldBacker<'T> list) (obs: NotifyBase) =
  +let mkCommand<'T> canExecuteHandler executeHandler (notifyOnFieldUpdate: IFieldBacker list) (obs: NotifyBase) =
       
       obs.MakeCommand<'T> (canExecuteHandler, executeHandler, notifyOnFieldUpdate)
   
  
positive.code: open System.ComponentModel
  open System.Windows.Input
  
  /// Implements INotifyPropertyChanged for use in observable models.
  type NotifyBase() =
  
      let propertyChanged = Event<_, _>()
      
      interface INotifyPropertyChanged with 
          [<CLIEvent>]
          member __.PropertyChanged = propertyChanged.Publish
  
  
negative.code:     let mutable value = match initialValue with
                          | Some t -> t
                          | None -> Unchecked.defaultof<'T>
      let internalUpdateEvent = new Event<obj>()
      let propertyName = 
          match propertyExpr with
          | PropertyGet(_, propOrValInfo, _) -> propOrValInfo.Name
  