anchor.diff:          override this.Correlation = correlation
   
           override this.Filter entityId =
  -            if refEq filter tautology then
  +            if Option.isNone filterOpt then
                   () // just pulls from lone associated store
               else
                   let indexOpt = store.TryIndexCorrelatedToI entityId
  
positive.code:             fn.Invoke (&array.[index], state)
  
      member this.IndexBuffered (entityId : uint64) (fn : Statement<'c, 'w>) world =
          if refEq filter tautology then
              store.WithCorrelatedsBuffered (fun aref world ->
                  let array = aref.Array
                  let index = store.IndexCorrelatedToI entityId
  
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 ()
  