code: private List<SimulatorEvent> processTaskAttemptCompletionEvent(
        TaskAttemptCompletionEvent event) {
      if (LOG.isDebugEnabled()) {
        LOG.debug("Processing task attempt completion event" + event);
      }
      
      long now = event.getTimeStamp();  
      TaskStatus finalStatus = event.getStatus();
      TaskAttemptID taskID = finalStatus.getTaskID();
      boolean killedEarlier = orphanTaskCompletions.remove(taskID);
      if (!killedEarlier) {
        finishRunningTask(finalStatus, now);
      }
      return SimulatorEngine.EMPTY_EVENTS;
    }
docs: Records that a task attempt has completed. Ignores the event for tasks
  that got killed after the creation of the completion event.
  
  @param event the TaskAttemptCompletionEvent the tracker sent to itself
  @return the list of response events, empty