anchor.diff:              | _ -> failwith ("Could not find expected system '" + systemName + "'.")
   
   /// A collection of subsystems that can be run in parallel.
  +/// NOTE: Because 'w is not a monoid (no mappend), null is passed in to all subsystems instead.
   type 'w SystemParallel () =
       inherit System<'w> ()
   
  
positive.code:     let systemsUnordered = dictPlus [] : Dictionary<string, 'w System>
      let systemsOrdered = List () : (string * 'w System) List
      let correlations = dictPlus [] : Dictionary<Guid, string List>
      let pipedValues = dictPlus [] : Dictionary<Guid, obj>
  
      member internal this.Correlations 
          with get () = correlations
  
      member this.RegisterPipedValue<'a> key (value : 'a) =
          pipedValues.[key] <- (value :> obj)
  
      member this.UnregisterPipedValue key =
          pipedValues.Remove key
  
      member this.TryIndexPipedValue<'a> key =
          match pipedValues.TryGetValue key with
          | (true, value) -> Some (value :?> 'a)
          | (false, _) -> None
  
      member this.IndexPipedValue<'a> key =
          pipedValues.[key] :?> 'a
  
  
negative.code: 
  namespace Nu
  open System
  open System.Collections.Concurrent
  open System.Collections.Generic
  open System.IO
  open System.Threading
  