anchor.diff:                Rotation = block.Actor.Rotation
                 Density = block.Density
                 BodyType = block.BodyType }
  -    { world2 with PhysicsMessages = bodyCreateMessage :: world2.PhysicsMessages }
  +    { world with PhysicsMessages = bodyCreateMessage :: world.PhysicsMessages }
   
   let addBlock address block world =
       let world2 = registerBlock address block world
  
positive.code:     | _ -> 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 addFeeler address feeler world =
      let world2 = registerFeeler address world
  
negative.code:     | _ -> failwith ("Expected MouseButtonData from address '" + str address + "'.")
  
  let registerToggle address world =
      let world2 = subscribe DownMouseLeftAddress address handleToggleEventDownMouseLeft world
      subscribe UpMouseLeftAddress address handleToggleEventUpMouseLeft world2
  
  let addToggle address toggle world =
      let world2 = registerToggle address world
  