docs: public HashIndex<E> unmodifiableView() {
      HashIndex<E> newIndex = new HashIndex<E>() {
        @Override
        public void unlock() { throw new UnsupportedOperationException("This is an unmodifiable view!"); }
        private static final long serialVersionUID = 3415903369787491736L;
      };
      newIndex.objects = objects;
      newIndex.indexes = indexes;
      newIndex.lock();
      return newIndex;
    }