interface ISleeper_v2 {
  default Sleeping doLater(long targetTime, Runnable r) {
    ret doLater(sysTimeToTimestamp(targetTime), r);
  }
  
  Sleeping doLater(Timestamp targetTime, Runnable r);
  
  public default Sleeping doAfter(double seconds, Runnable r) {
    ret doLater(tsNow().plusSeconds(seconds), r);
  }
}