datatype readiness a = Ready of a | Waiting | Invalid of string
datatype rpcResult a = Success of a | Failure of string

fun warning [a] (s : signal (readiness a)) =
    <xml><dyn signal={v <- s;
                      return (case v of
                                  Invalid s => <xml><b>{[s]}</b></xml>
                                | _ => <xml/>)}/></xml>