code: private <T> ForkJoinTask<T> externalSubmit(ForkJoinTask<T> task) {
          Thread t; ForkJoinWorkerThread w; WorkQueue q;
          if (task == null)
              throw new NullPointerException();
          if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) &&
              (w = (ForkJoinWorkerThread)t).pool == this &&
              (q = w.workQueue) != null)
              q.push(task);
          else
              externalPush(task);
          return task;
      }
docs: Pushes a possibly-external submission.