anchor.diff:      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>
  +    let pipedValues = ConcurrentDictionary<Guid, obj> ()
   
       member internal this.Correlations 
           with get () = correlations
   
  +    /// Thread-safe.
       member this.RegisterPipedValue<'a> key (value : 'a) =
  -        pipedValues.[key] <- (value :> obj)
  -
  +        pipedValues.[key] <- value :> obj
  +
  +    /// Thread-safe.
       member this.UnregisterPipedValue key =
  -        pipedValues.Remove key
  -
  +        pipedValues.TryRemove key
  +
  +    /// Thread-safe.
       member this.TryIndexPipedValue<'a> key =
           match pipedValues.TryGetValue key with
           | (true, value) -> Some (value :?> 'a)
           | (false, _) -> None
   
  +    /// Thread-safe.
       member this.IndexPipedValue<'a> key =
           pipedValues.[key] :?> 'a
   
  
positive.code:             | _ -> failwith ("Could not find expected system '" + systemName + "'.")
  
  /// A collection of subsystems that can be run in parallel.
  type 'w SystemParallel () =
      inherit System<'w> ()
  
  
negative.code: 
  namespace Nu
  open System
  open System.Collections.Concurrent
  open System.Collections.Generic
  open System.IO
  open System.Threading
  