In the online version of the book there a whole bunch of within-glossary gref links to other glossary items that don't exist (e.g. python, r_language, package, etc).
Note that this is only an issue for the online version of the book, because in the PDF version within-glossary references to other glossary items are not highlighted.
In the online version of the book there a whole bunch of within-glossary gref links to other glossary items that don't exist (e.g. python, r_language, package, etc).