anchor.diff:                          Trace.TraceInformation("Reactor: {0:n3} --> {1}.{2} ({3}), remaining {4}", DateTime.Now.TimeOfDay.TotalSeconds, userOpName, opName, opArg, inbox.CurrentQueueLength)
                           let time = Stopwatch()
                           time.Start()
  -
  -                        threadsUsed.[currentThread.ManagedThreadId] <- 0uy
                           op ctok
  -                        let res, _ = threadsUsed.TryRemove(currentThread.ManagedThreadId)
  -                        assert res
  -
                           time.Stop()
                           let span = time.Elapsed
                           //if span.TotalMilliseconds > 100.0 then 
  
positive.code:                                             pauseBeforeBackgroundWork
                                      return! inbox.TryReceive(timeout) }
  
                      let currentThread = Thread.CurrentThread
                      currentThread.CurrentUICulture <- culture
  
                      match msg with
                      | Some (SetBackgroundOp bgOpOpt) -> 
  
negative.code:     let mutable bgOpCts = new CancellationTokenSource()
  
      let threadsUsed = System.Collections.Concurrent.ConcurrentDictionary<int, byte>()
  
      let useThread (thread: Thread) =
          thread.CurrentUICulture <- culture
          threadsUsed.[thread.ManagedThreadId] <- 0uy
          { new IDisposable with 
              member _.Dispose() = 
                  let res, _ = threadsUsed.TryRemove(thread.ManagedThreadId)
                  assert res
          }
  
      /// Mailbox dispatch function.
      let builder = 
  