code: final boolean apparentlyFirstQueuedIsExclusive() {
          Node h, s;
          return (h = head) != null &&
              (s = h.next)  != null &&
              !s.isShared()         &&
              s.thread != null;
      }
docs: Returns {@code true} if the apparent first queued thread, if one
  exists, is waiting in exclusive mode.  If this method returns
  {@code true}, and the current thread is attempting to acquire in
  shared mode (that is, this method is invoked from {@link
  #tryAcquireShared}) then it is guaranteed that the current thread
  is not the first queued thread.  Used only as a heuristic in
  ReentrantReadWriteLock.