pipelets loop option2

pipelets loop option2.fs

    let mailbox = MailboxProcessor.Start(fun inbox ->
        let rec loop routes = async {
            let! msg = inbox.Receive()
            match msg with
            | Payload(data) ->
                ss.Release() |> ignore
                let result = compute data routes |> Async.Catch |> Async.RunSynchronously
                match result with
                | Choice1Of2 _ -> ()
                | Choice2Of2 exn -> errors exn
                return! loop routes                    
            | Attach(stage) -> return! loop (stage::routes)
            | Detach(stage) -> return! loop (List.filter (fun x -> x <> stage) routes)
        }
        loop [])

What's the MSDN reference for this implementation?