@Override
 public ImmutableBitmap difference(ImmutableBitmap otherBitmap) {
   WrappedBitSetBitmap retval = new WrappedBitSetBitmap((BitSet) bitmap.clone());
   retval.andNot((WrappedBitSetBitmap) otherBitmap);
   return retval;
 }
 @Override
 public ImmutableBitmap intersection(ImmutableBitmap otherBitmap) {
   WrappedBitSetBitmap retval = new WrappedBitSetBitmap((BitSet) bitmap.clone());
   retval.and((WrappedBitSetBitmap) otherBitmap);
   return retval;
 }