code: protected static ForkJoinTask<?> pollTask() {
      Thread t;
      ForkJoinWorkerThread wt;
      return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
          ? (wt = (ForkJoinWorkerThread) t).pool.nextTaskFor(wt.workQueue)
          : null;
    }
docs: If the current thread is operating in a ForkJoinPool, unschedules and returns, without
  executing, the next task queued by the current thread but not yet executed, if one is
  available, or if not available, a task that was forked by some other thread, if available.
  Availability may be transient, so a {@code null} result does not necessarily imply quiescence
  of the pool this task is operating in. 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