code: private synchronized void retireReduce(TaskInProgress tip) {
      if (runningReduces == null) {
        LOG.warn("Running list for reducers missing!! "
                 + "Job details are missing.");
        return;
      }
      runningReduces.remove(tip);
    }
docs: Remove a reduce TIP from the list for running-reduces
  Called when a reduce fails/completes
  @param tip the tip that needs to be retired