Just after I got the initial Undo working, I did something bad to my neck. I had terrible pain in my neck and shoulder, and my left arm was partially paralyzed. I was laid up for about a month. Between the drugs and the pain, it was difficult to make progress on the book, but I was able to do what seemed like some pretty good paper design.
In fact, I think the paper design ideas were good but that they gave me too much confidence and induced me to take approaches that were too complex to implement in a half-day or so. My overall productivity drops rapidly if I work in bites bigger than that, and this was no exception. I bogged down, failed to notice mistakes in the original paper design, made new mistakes, and foundered again and again.