. . . so far. But I skipped the entire section on interval arithmetic and especially Ex. 2.16, in view of the authors' warning). I enjoyed, by the way, Exercise 2.5, where you have to represent pairs as integers. It reminded me of an interesting chapter in a book called "What Is Mathematics?" by Courant and Robbins. In that book, there is a description of Cantor's proofs of the "denumerability" on the "rational" numbers, and the "non-denumerability" of the "real" numbers. Some of the other chapters of that book also seem to me to related to topics in SICP, for example the "continued fractions" of Exercise 1.37.