anchor.diff:  
           // track registration
           if registered then
  -            registrations.Add entityId |> ignore<bool>
  -            unregistrations.Remove entityId |> ignore<bool>
  +            changes.Add struct (entityId, true)
   
           // register correlation
           if registered && not isolated then
  
positive.code:         systemTree |> ListTree.map (fun system -> system.WithCorrelatedsBuffered fn worldOpt)
  
      member this.PopHierarchyChanges () =
          let (rs, us) = (registrations, unregistrations)
          registrations <- hashSetPlus<Guid> HashIdentity.Structural []
          unregistrations <- hashSetPlus<Guid> HashIdentity.Structural []
          (rs, us)
  
      member this.IndexNode nodeId =
          match systemDict.TryGetValue nodeId with
  
negative.code: 
      let systemTree = ListTree.makeEmpty<SystemCorrelated<'c, 'w>> ()
      let systemDict = dictPlus<Guid, SystemCorrelated<'c, 'w>> HashIdentity.Structural []
      let mutable changes = List<struct (Guid * bool)> ()
  
      new (ecs) = SystemHierarchical (false, false, ecs)
  
  