[drostie]

I would guess that it's something like SICP introducing assignment halfway through the book, dramatic, unexpected, and you've been using a weaker version for many chapters now. (Chapter 6 is about recursive data types, and comes immediately after chapter 5's induction to give some perspective.) But it might also be because, as an introductory mathematics course, it wants to stay a good distance away from algorithms and messy concepts like big-O notation.