docs: private void writeObject(java.io.ObjectOutputStream s)
          throws java.io.IOException {
  
          // Write out any hidden stuff
          s.defaultWriteObject();
  
          // Write out all elements in the proper order.
          for (Node<E> p = first(); p != null; p = succ(p)) {
              final E item;
              if ((item = p.item) != null)
                  s.writeObject(item);
          }
  
          // Use trailing null as sentinel
          s.writeObject(null);
      }