sclass MultiSleeper is ISleeper, AutoCloseable {
  MultiSetMap<Timestamp, Runnable> entries = treeMultiSetMap();
  new RestartableCountdown countdown;
  
  private void check {
    var time = nextWakeUpTime();
    var action = firstValue(entries);
    countdown.setTargetTime(time == null ? 0 : time.sysTime(), r {
      Set<Runnable> toCall;
      synchronized(MultiSleeper.this) {
        toCall = entries.get(time);
        entries.remove(time);
        check();
      }
      pcallFAll(toCall);
    });
  }
  
  private synchronized void removeEntry(Timestamp targetTime, Runnable action) {
    entries.remove(targetTime, action);
  }
  
  // API
  
  synchronized Timestamp nextWakeUpTime() {
    ret firstKey(entries);
  }
  
  public synchronized Sleeping doLater(Timestamp targetTime, Runnable r) {
    if (r == null || targetTime == null) null;
    targetTime = max(targetTime, tsNow());
    entries.put(targetTime, r);
    check();
    ret new Sleeping(targetTime, r) {
      close {
        removeEntry(targetTime, r);
      }
    };
  }
  
  close {
    dispose countdown;
  }
}