anchor.diff:                Rotation = avatar.Actor.Rotation
                 Density = avatar.Density
                 BodyType = BodyType.Dynamic }
  -    { world2 with PhysicsMessages = bodyCreateMessage :: world2.PhysicsMessages }
  +    { world with PhysicsMessages = bodyCreateMessage :: world.PhysicsMessages }
   
   let addAvatar address avatar world =
       let world2 = registerAvatar address avatar world
  
positive.code:         let w_ = unsubscribe TickAddress [] w_
          let w_ = unsubscribe ClickButtonAddress [] w_
          let w_ = set None w_ World.optActiveScreenAddress
  
          let w_ = removeEntityModelsFromGroup address w_
          set None w_ (World.optGroupModel address)
  
  
negative.code:     set None world2 (World.optGroupModel address)
  
  // TODO: see if there's a nice way to put this module in another file
  [<RequireQualifiedAccess>]
  module Test =
  
      let ScreenAddress = [Lun.make "testScreen"]
  