code: public static int andNotCardinality(final ImmutableRoaringBitmap x1,
                                        final ImmutableRoaringBitmap x2) {
      return x1.getCardinality() - andCardinality(x1, x2);
    }
docs: Cardinality of the bitwise ANDNOT (left difference) operation.
  The provided bitmaps are *not* modified. This operation is thread-safe
  as long as the provided bitmaps remain unchanged.
  
  @param x1 first bitmap
  @param x2 other bitmap
  @return cardinality of the left difference