anchor.diff:  
       type Entity with
           
  -        // button
  +        (* button xfields *)
           member this.IsDown with get () = this?IsDown () : bool
           member this.SetIsDown (value : bool) : Entity = this?IsDown <- value
           member this.UpSprite with get () = this?UpSprite () : Sprite
  
positive.code:         member this.ToggleSound with get () = this?ToggleSound () : Sound
          member this.SetToggleSound (value : Sound) : Entity = this?ToggleSound <- value
  
          // feeler
          member this.IsTouched with get () = this?IsTouched () : bool
          member this.SetIsTouched (value : bool) : Entity = this?IsTouched <- value
  
          // block
          member this.PhysicsId with get () = this?PhysicsId () : PhysicsId
          member this.SetPhysicsId (value : PhysicsId) : Entity = this?PhysicsId <- value
          member this.Density with get () = this?Density () : single
  
negative.code:                     else (message, keepRunning, world')
                  else (message, true, world)
              | _ -> failwith ("Expected MouseButtonData from event '" + addrToStr event + "'.")
  
          override this.Init (button, dispatcherContainer) =
              let button' = base.Init (button, dispatcherContainer)
              ((((button'
  