public boolean contains(@Nullable Object object) {
        return set1.contains(object) && set2.contains(object);
      }

      @Override
      public boolean containsAll(Collection<?> collection) {
        return set1.containsAll(collection) && set2.containsAll(collection);
      }

      @Override
      int minSize() {
        return 0;
      }

      @Override
      int maxSize() {