code: public <T> ForkJoinTask<T> submit(ForkJoinTask<T> task) {
          if (task == null)
              throw new NullPointerException();
          doSubmit(task);
          return task;
      }
docs: Submits a ForkJoinTask for execution.
  
  @param task the task to submit
  @return the task
  @throws NullPointerException if the task is null
  @throws RejectedExecutionException if the task cannot be
  scheduled for execution