anchor.diff:      // OPTIMIZATION: cache one entity change address to reduce allocation where possible.
       let mutable changeEventNamesFree = true
       let changeEventNamesCached = [|"Change"; ""; "Event"; ""; ""; ""|]
  -
  -    // OPTIMIZATION: cache empty children
  -    let emptyChildrenImperative = USet.makeEmpty HashIdentity.Structural Imperative
  -    let emptyChildrenFunctional = USet.makeEmpty HashIdentity.Structural Functional
   
       type World with
   
  
positive.code: 
          static member internal getEntityChildren entity world =
              match world.EntityHierarchy.TryGetValue entity with
              | (true, children) -> children
              | (false, _) ->
                  match World.getCollectionConfig world with
                  | Imperative -> emptyChildrenImperative
                  | Functional -> emptyChildrenFunctional
  
          static member internal getEntityDescendants entity world =
              seq {
  
negative.code: 
          static member internal traverseEntityChildren effect entity (world : World) =
              let children = World.getEntityChildren entity world
              Seq.fold (fun world child -> effect entity child world) world children
  
          static member internal traverseEntityDescendants effect entity (world : World) =
              let children = World.getEntityChildren entity world
              let world = Seq.fold (fun world child -> effect entity child world) world children
              Seq.fold (fun world child -> World.traverseEntityDescendants effect child world) world children
  
          static member internal setEntityParentOpt value entity world =
              let newParentOpt = value
  