sclass MultiSleeper extends RestartableCountdown is ISleeper_v2 {
  new TreeMultiMap<Timestamp, Runnable> entries;
  
  void check {
    var time = nextWakeUpTime();
    var action = firstValue(entries);
    setTargetTime(time == null ? 0 : time.sysTime(), r {
      L<Runnable> toCall;
      synchronized(MultiSleeper.this) {
        toCall = entries.get(time);
        entries.remove(time);
      }
      check();
      pcallFAll(toCall);
    });
  }
  
  // API
  
  synchronized Timestamp nextWakeUpTime() {
    ret firstKey(entries);
  }
  
  public synchronized Sleeping doLater(Timestamp targetTime, Runnable r) {
    if (r == null || targetTime == null) null;
    entries.put(targetTime, r);
    check();
  }
}