code: protected static ForkJoinTask<?> pollSubmission() {
      Thread t;
      return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
          ? ((ForkJoinWorkerThread) t).pool.pollSubmission()
          : null;
    }
docs: If the current thread is operating in a ForkJoinPool, unschedules and returns, without
  executing, a task externally submitted to the pool, if one is available. Availability may be
  transient, so a {@code null} result does not necessarily imply quiescence of the pool. This
  method is designed primarily to support extensions, and is unlikely to be useful otherwise.
  
  @return a task, or {@code null} if none are available
  @since 9