Thanks for reporting the issue! I’ve put it into the issue tracker, I’ll try to clean it up for beta 3. There’s a note in the preface about source examples advancing through the chapter, but it’s not as clear as I’d like it to be - so I’ll see if I can make it more explicit in the body text. The “embed source” feature of the book builder is awesome in general, but does make it tricky when a chapter needs to work over multiple examples (I wanted to provide the reader with source for steps along the way, rather than a big dump of the final product of each chapter; the journey is more important than the destination for learning a language).
I’m not sure if the system will let me change the rendering of the source names, so it’ll probably take the form of a note that it’s advanced to the next section. (I’ll also triple-check that I intended to advance at each point).
Thanks - beta 3 will clear this up. 