code: final boolean tryExternalUnpush(ForkJoinTask<?> task) {
          int r = ThreadLocalRandom.getProbe();
          WorkQueue[] ws; WorkQueue w; int wl;
          return ((ws = workQueues) != null &&
                  (wl = ws.length) > 0 &&
                  (w = ws[(wl - 1) & r & SQMASK]) != null &&
                  w.trySharedUnpush(task));
      }
docs: Performs tryUnpush for an external submitter.