anchor.diff:      /// event will be confirmed.
       /// </summary>
       abstract DeferEvent: ('Event -> 'State) -> 'Event seq -> unit
  +    
  +    /// <summary>
  +    /// Defers a callback function to be called after persisting events will be confirmed.
  +    /// </summary>
  +    abstract DeferAsync: (obj -> unit) -> obj -> unit
   
       /// <summary>
       /// Returns currently attached journal actor reference.
  
positive.code:             member __.Log = lazy (Akka.Event.Logging.GetLogger(context)) 
              member __.Defer fn = deferables <- fn::deferables
              member __.DeferEvent callback events = events |> Seq.iter (fun e -> this.DeferAsync(e, Action<_>(updateState callback)))
              member __.PersistEvent callback events = this.PersistAll(events, Action<_>(updateState callback))
              member __.AsyncPersistEvent callback events = this.PersistAllAsync(events, Action<_>(updateState callback)) 
              member __.Journal() = this.Journal
  
negative.code:             member __.Log = lazy (Akka.Event.Logging.GetLogger(context)) 
              member __.Defer fn = deferables <- fn::deferables
              member __.DeferEvent callback events = events |> Seq.iter (fun e -> this.DeferAsync(e, Action<_>(updateState callback)))
              member __.DeferAsync callback obj = this.DeferAsync(obj, Action<_>(callback))
              member __.PersistEvent callback events = this.PersistAll(events, Action<_>(updateState callback))
              member __.AsyncPersistEvent callback events = this.PersistAllAsync(events, Action<_>(updateState callback)) 
              member __.Journal() = this.Journal
  