This is how you serve science. Thanks to Tom Leinster. "This electronic version is not only free; it is also freely editable. For instance, if you would like to teach a course using this book but some of the examples are unsuitable for your class, you can remove them or add your own. Similarly, if there is notation that you dislike, you can easily change it; or if you want to reformat the text for reading on a particular device, that is easy too."