[cldr]

They can do that. It's plain on any good documentation site (e.g. http://en.cppreference.com/w/cpp/string/basic_string) that `string::size_type` is unsigned. The teachers just have to be knowledgeable enough to let students know of good documentation (if, by documentation, you mean what I take you to mean, which is stuff other than the standard itself, which is nearly unintelligible for new people). The technique I gave was just for fun.