anchor.diff:      let mutable currentProgram : Option<AdaptiveCode> = None
       let mutable currentChanger = Mod.constant FrameStatistics.Zero
       let mutable frag : 'f = match precompiled with | Some p -> p | None -> null
  -    let mutable lastSurface = None
  +    let mutable lastPrev = None
   
       let hasProgram =
           match precompiled with
  
positive.code:                 | null -> PreparedRenderObject.Empty
                  | _ -> prev.RenderObject
  
          let mySurface = rj.Original.Surface.GetValue()
  
          match lastSurface with
              | Some s when s = mySurface -> FrameStatistics.Zero
              | _ ->
                  lastSurface <- Some mySurface
  
                  match frag with
                      | null -> frag <- ctx.handler.Create []
  
negative.code: 
          currentChanger.RemoveOutput changer
          currentChanger <- Mod.constant FrameStatistics.Zero
          lastPrev <- None
          prev <- null
          next <- null
  
  