anchor.diff:  
       /// 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"
   
  
positive.code: 
          static member internal synchronizeSimulants contentMapper contentKey mapGeneralized current origin owner parent world =
              let previous =
                  match World.tryGetKeyedValueFast<IComparable LensComparable USet> (contentKey, world) with
                  | (false, _) -> emptyPreviousSimulants
                  | (true, previous) -> previous
              let added = USet.differenceFast current previous
  
negative.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 // ELMISH_CACHE
                  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
  