The author hacks it in using <i>const_cast</i> (<a href="https://en.cppreference.com/w/cpp/language/const_cast" rel="nofollow">https://en.cppreference.com/w/cpp/language/const_cast</a>), but doesn’t seem to know <i>mutable</i> (<a href="https://en.cppreference.com/w/cpp/language/cv#mutable" rel="nofollow">https://en.cppreference.com/w/cpp/language/cv#mutable</a>) exists for this purpose.