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;

  }

}