anchor.diff:  
       member this.IndexSystems entityId =
           match correlations.TryGetValue entityId with
  -        | (true, systems) -> seq systems
  -        | (false, _) -> Seq.empty
  +        | (true, systems) -> systems
  +        | (false, _) -> emptySystems
   
       member this.SubscribePlus<'d> subscriptionId eventName (callback : SystemCallback<'d, 'w>) =
           match systemSubscriptions.TryGetValue eventName with
  
positive.code:             | Some system ->
                  if system.UnregisterMultiplexed simplexName entityId this then
                      match this.Correlations.TryGetValue entityId with
                      | (true, correlation) -> correlation.Add system
                      | (false, _) -> this.Correlations.Add (entityId, List [system :> 'w System])
              | _ -> failwith ("Could not find expected system '" + Unchecked.defaultof<'c>.TypeName + "'.")
  
  /// An Ecs system that stores components in a tree hierarchy.
  
negative.code:             | Some system ->
                  if system.UnregisterHierarchical nodeId entityId this then
                      match this.Correlations.TryGetValue entityId with
                      | (true, correlation) -> correlation.Remove system.Name |> ignore<bool>
                      | (false, _) -> ()
                      true
                  else false
              | _ -> failwith ("Could not find expected system '" + Unchecked.defaultof<'c>.TypeName + "'.")
  
  /// A correlated entity reference.
  