docs: public static int orCardinality(final ImmutableRoaringBitmap x1,
        final ImmutableRoaringBitmap x2) {
      // we use the fact that the cardinality of the bitmaps is known so that
      // the union is just the total cardinality minus the intersection
      return x1.getCardinality() + x2.getCardinality() - andCardinality(x1, x2);
    }