An apology. On July 19th, 2016, my internet colleague Musa Al-hassy pointed out that I had made a serious error in this JAW by not stipulating that
Q is idempotent. It took me close to four years to fix the error. Thank you, Musa. In my defense, all I can say is: kids, man. (
End of An apology.)
—from
http://mathmeth.com/jaw/main/jaw1xx/jaw100.pdf