anchor.diff:      let cells = Array.zeroCreate nCells
       let before = GC.GetTotalMemory true
       for i=0 to nCells-1 do
  -      cells.[i] <- create i
  +      cells.[i] <- cell i
       printf "%4d b/c " (max 0L (GC.GetTotalMemory true - before) / int64 nCells)
       seq {1 .. nJobs}
       |> Seq.map (fun _ -> async {
  
positive.code: 
    type Cell<'a> = MailboxProcessor<Request<'a>>
  
    let create (x: 'a) : Cell<'a> = MailboxProcessor.Start <| fun inbox ->
      let rec server x = async {
        let! req = inbox.Receive ()
        match req with
  
negative.code:   let get (c: Cell<'a>) : Job<'a> = Ch.take c.getCh
    let put (c: Cell<'a>) (x: 'a) : Job<unit> = Ch.give c.putCh x
  
    let cell (x: 'a) : Job<Cell<'a>> = Job.delay <| fun () ->
      let c = {getCh = Ch.Now.create (); putCh = Ch.Now.create ()}
      Job.start
       (Job.iterate x (fun x ->
         Alt.pick ((Ch.Alt.take c.putCh)
               <|> (Ch.Alt.give c.getCh x >-> fun () -> x)))) >>% c
  
    let run nCells nJobs nUpdates =
      printf "HopacAlt: "
  