Really good for mathematical proofs. Needs a little more explanation about set theory and how x and y can be moved across sets A and B because it seems vague. Also, needs to clearly state that if any number x is even, then x = 2k where k is an integer. Also if any number y is odd, then y = 2k + 1 where k is an integer. That wasn't really as clearly explained as I would have liked it to be.