anchor.diff:                              let mapGeneralized = Lens.getWithoutValidation binding.CBSource world
                               let currentKeys = mapGeneralized.Keys
                               let previousKeys =
  -                                match World.tryGetKeyedValueFast<IComparable List> (contentKeyPlus1, world) with
  -                                | (false, _) -> List ()
  +                                match World.tryGetKeyedValueFast<IComparable List> (contentKeyPlus1, world) with // ELMISH_CACHE
  +                                | (false, _) -> List () // TODO: use cached module binding of empty List.
                                   | (true, previous) -> previous
                               if not (currentKeys.SequenceEqual previousKeys) then
                                   let world = World.addKeyedValue contentKeyPlus1 currentKeys world
                                   let current = World.makeLensesCurrent currentKeys binding.CBSource world
                                   World.synchronizeSimulants binding.CBMapper binding.CBContentKey mapGeneralized current binding.CBOrigin binding.CBOwner binding.CBParent world
                               else world
  -                        else 
  +                        else
                               let config = World.getCollectionConfig world
                               let lensesCurrent = USet.makeEmpty (LensComparer ()) config
                               let world = World.synchronizeSimulants binding.CBMapper binding.CBContentKey (MapGeneralized.make Map.empty) lensesCurrent binding.CBOrigin binding.CBOwner binding.CBParent world
  
positive.code:             if changed then
                  // HACK: we need to use content key for an additional purpose, so we add 2 (key with add 1 may already be used in invoking function).
                  // TODO: make sure our removal of this key's entry is comprehensive or has some way of not creating too many dead entries.
                  let contentKeyPlus2 = Gen.idDeterministic 2 contentKey
                  let world = World.removeSynchronizedSimulants contentKeyPlus2 removed world
                  let world = World.addSynchronizedSimulants contentMapper contentKeyPlus2 mapGeneralized added origin owner parent world
                  let world = World.addKeyedValue contentKey current world
  
negative.code: 
      /// The default screen - may or may not exist.
      let DefaultScreen = Screen "Screen"
  
      /// The default group - may or may not exist.
      let DefaultGroup = DefaultScreen / "Group"
  
      /// The default entity - may or may not exist.
      let DefaultEntity = DefaultGroup / "Entity"
  
  