9/10/19: The SL version number is updated from 8.1 to 8.2. Main structure remains the same. But there are enough changes to mark this as a substantive improvement over previous versions.
4/29/19: The SL version number is incremented from 8.0 to 8.1. Changes to the main body are minor. The primary update is that Answers to Selected Exercises are moved online. This permits large pages and so improved formatting.