anchor.diff:              fn.Invoke (&array.[index], state)
   
       member this.IndexBuffered (entityId : uint64) (fn : Statement<'c, 'w>) world =
  -        if refEq filter tautology then
  +        if Option.isNone filterOpt then
               store.WithCorrelatedsBuffered (fun aref world ->
                   let array = aref.Array
                   let index = store.IndexCorrelatedToI entityId
  
positive.code:                 world
  
      member this.Index (entityId : uint64) (fn : Statement<'c, 's>) state =
          if refEq filter tautology then
              let cref = store.IndexCorrelated entityId
              fn.Invoke (&cref.Index, state)
          else
  
negative.code: 
  /// An ECS system.
  type System<'c, 'w when
      'c : struct and 'c :> 'c Component> (filterOpt : (uint64 -> bool) option, ecs : 'w Ecs) =
      
      let filter = Option.getOrDefault tautology filterOpt
      let cache = OrderedDictionary<uint64, int> HashIdentity.Structural
      let correlation = hashSetPlus<string> StringComparer.Ordinal [typeof<'c>.Name]
      let store = ecs.IndexStore<'c, CorrelatedStore<'c, 'w>> ()
      let allocated = HashSet<uint64> HashIdentity.Structural
      let unallocated = HashSet<uint64> HashIdentity.Structural
  
      new (ecs) = System<_, _> (None, ecs)
  
      member this.Cached = cache.Keys
      member this.Allocated = allocated.AsReadOnly ()
  