anchor.diff:      let mutable freeIndex = 0
       let freeList = Queue<int> ()
       let correlations = dictPlus [] : Dictionary<Guid, int>
  -    
  +
       member this.Components with get () = components and internal set value = components <- value
       member this.FreeIndex with get () = freeIndex and internal set value = freeIndex <- value
       member internal this.FreeList with get () = freeList
       member internal this.Correlations with get () = correlations
   
  -    member this.HasComponent entityId =
  +    member this.CorrelateEntities () =
  +        correlations.Keys :> _ IEnumerable
  +
  +    member this.QualifyCorrelated entityId =
           correlations.ContainsKey entityId
   
  -    member this.GetComponentIndex entityId =
  +    member this.IndexCorrelatedI entityId =
           let (found, index) = correlations.TryGetValue entityId
           if not found then raise (InvalidOperationException "entityId")
           index
   
  -    member this.GetComponent entityId =
  -        let index = this.GetComponentIndex entityId
  +    member this.IndexCorrelated entityId =
  +        let index = this.IndexCorrelatedI entityId
           &components.[index]
  -
  -    member this.GetEntities () =
  -        correlations.Keys :> _ IEnumerable
   
       abstract member RegisterCorrelated : Guid -> 't -> 'w Ecs -> int
       default this.RegisterCorrelated entityId comp _ =
  
positive.code:             and set value = this.RefCount <- value
      member this.Components
          with get () = this.Entries.Values :> 't Simplex IEnumerable
      member this.RegisterComponent (multiId, comp) =
          this.Entries.Add (multiId, { Simplex = comp })
      member this.UnregisterComponent multiId =
          this.Entries.Remove multiId
  
  /// A base system type of an Ecs.
  
negative.code:             match this.TryIndexSystem<SystemCorrelated<'t, 'w>> systemName with
              | Some system ->
                  if system.UnregisterCorrelated entityId this then
                      match this.Correlations.TryGetValue entityId with
                      | (true, correlation) -> correlation.Add systemName
                      | (false, _) -> this.Correlations.Add (entityId, List [systemName])
  