return x;
sort(dummies_of_factor.begin(), dummies_of_factor.end(), ex_is_less());
ex new_factor = rename_dummy_indices_uniquely(used_indices,
return x;
sort(dummies_of_factor.begin(), dummies_of_factor.end(), ex_is_less());
ex new_factor = rename_dummy_indices_uniquely(used_indices,