anchor.diff:        async {
           let! msg = self.Receive ()
           match msg with
  +        | Worker.Retire -> return ()
           | Worker.Do command ->
             match command with
             | Process uri -> doit uri
  
positive.code: 
  type Worker =
    | Do of Command
  
  type Manager =
    | Do of int * Command
  
negative.code:             return! loop status
            | Running ->
              let workers = [ 1 .. count ] |> List.map (fun _ -> newWorker self)
              workers |> List.iter (fun worker -> worker.Post(Worker.Do(command)); worker.Post(Retire))
              return! loop status
          | Manager.Stop ->
            return! loop Stopped
  