code: 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);
    }
docs: Cardinality of the bitwise OR (union) operation. The provided bitmaps are *not* modified. This
  operation is thread-safe as long as the provided bitmaps remain unchanged.
  
  If you have more than 2 bitmaps, consider using the FastAggregation class.
  
  @param x1 first bitmap
  @param x2 other bitmap
  @return cardinality of the union
  @see BufferFastAggregation#or(ImmutableRoaringBitmap...)
  @see BufferFastAggregation#horizontal_or(ImmutableRoaringBitmap...)