docs: public boolean contains(Object o) {
          if (o == null) return false;
          final ReentrantLock lock = this.lock;
          lock.lock();
          try {
              for (Node<E> p = first; p != null; p = p.next)
                  if (o.equals(p.item))
                      return true;
              return false;
          } finally {
              lock.unlock();
          }
      }