anchor.diff:          let tmxMap = TmxMap "TileMap.tmx"
           TileMap
               { Actor = makeDefaultActor ()
  +              PhysicsIds = []
                 Density = NormalDensity
                 TileMapAsset = { TileMapAssetName = Lun.make "TileMap"; PackageName = Lun.make "Default"; PackageFileName = "AssetGraph.xml" }
                 TmxMap = tmxMap
  
positive.code:       
  type [<StructuralEquality; NoComparison; CLIMutable>] TileMap =
      { Actor : Actor
        //PhysicsIds : Id list
        Density : single
        TileMapAsset : TileMapAsset
        TmxMap : TmxMap
  
negative.code: let entityModelOptActor =
      { Get = fun this ->
          match this with
          | Button _
          | Label _
          | TextBox _
          | Toggle _
          | Feeler _ -> None
          | Block block -> Some block.Actor
          | Avatar avatar -> Some avatar.Actor
          | TileMap tileMap -> Some tileMap.Actor
        Set = fun optActor this ->
          let actor = Option.get optActor
          match this with
          | Button _
          | Label _
          | TextBox _
          | Toggle _
          | Feeler _ -> failwith "EntityModel is not an actor."
          | Block block -> Block { block with Actor = actor }
          | Avatar avatar -> Avatar { avatar with Actor = actor }
          | TileMap tileMap -> TileMap { tileMap with Actor = actor }}
  