Unexpected behaviour of `create_sigma_star_nft`

When running the following:

```
Nft s {nft::builder::create_sigma_star_nft(2)};
std::cout << s.is_in_lang({'a', 'b'}) << std::endl;
```

the output is `0`, which is unexpected. It seems that the reason is that `is_in_lang` does not correctly handle `DONT_CARE` symbols, but I am not sure about that.