anchor.diff:      member this.RegisterSubsystem subsystemName subsystem =
           subsystems.[subsystemName] <- subsystem
   
  -    member this.UnregisterSubsystem subsystemName =
  +    member this.UnregisterSubsystem subsystemName (_ : 'w System) =
           subsystems.Remove subsystemName
   
       override this.ProcessUpdate ecs world =
  
positive.code: 
  /// A collection of systems that can be run in parallel.
  /// Because 'w is not a monoid (no mappend), null is passed in to all subsystems. Any information that affects the
  /// world has to use piped values and the related process lambda.
  type 'w SystemParallel () =
      inherit System<'w> ()
  
  
negative.code:     member this.RegisterSystem systemName system =
          systemsUnordered.Add (systemName, system)
          systemsOrdered.Add (systemName, system)
          //system.RegisterPipedValue this
  
      member this.UnregisterSystem systemName (_ : 'w System) =
          //system.UnregisterPipedValue this
          systemsOrdered.RemoveAll (fun (systemName', _) -> systemName' = systemName) |> ignore
          systemsUnordered.Remove systemName |> ignore
  
  