anchor.diff:      | _ -> failwith ("Expected MouseButtonData from address '" + str address + "'.")
       
   let registerFeeler address world =
  -    let optOldFeeler = get world (World.optFeeler address)
  -    let world_ = if optOldFeeler.IsSome then unregisterFeeler address world else world
  -    let world_ = subscribe DownMouseLeftAddress address handleFeelerEventDownMouseLeft world_
  -    subscribe UpMouseLeftAddress address handleFeelerEventUpMouseLeft world_
  +    let world2 = subscribe DownMouseLeftAddress address handleFeelerEventDownMouseLeft world
  +    subscribe UpMouseLeftAddress address handleFeelerEventUpMouseLeft world2
   
   let addFeeler address feeler world =
       let world2 = registerFeeler address world
  
positive.code:               Rotation = actor.Rotation
                Density = tileMap.Density
                BodyType = BodyType.Static }
      { world2 with PhysicsMessages = bodyCreateMessage :: world2.PhysicsMessages }*)
      world2
  
  let addTileMap address tileMap world =
      let world2 = registerTileMap address tileMap world
  
negative.code:               Rotation = avatar.Actor.Rotation
                Density = avatar.Density
                BodyType = BodyType.Dynamic }
      { world with PhysicsMessages = bodyCreateMessage :: world.PhysicsMessages }
  
  let addAvatar address avatar world =
      let world2 = registerAvatar address avatar world
  