The author hacks it in using const_cast (https://en.cppreference.com/w/cpp/language/const_cast https://en.cppreference.com/w/cpp/language/const_cast), but doesn

The author hacks it in using const_cast (https://en.cppreference.com/w/cpp/language/const_cast https://en.cppreference.com/w/cpp/language/const_cast), but doesn’t seem to know mutable (https://en.cppreference.com/w/cpp/language/cv#mutable https://en.cppreference.com/w/cpp/language/cv#mutable) exists for this purpose.