That Logic class for which I’m a TA has just started practicing derivations in sentential logic, so for the past week or so I’ve been doing some of the harder ones from the textbook just to make sure I’m not stumped when a student asks me to go over something during my office hours. I was pleasantly surprised to find that going a year without touching these things hasn’t left me unable to do them, but yesterday afternoon I spent about half an hour puzzling over one particular problem. I tried every trick I could remember, but I’d still end up two or three subderivations deep with no forseeable way out. Finally, I decided to dig up my notebook from when I took this very same class, knowing that somewhere in it I would find my solution to this very same derivation. After weeding through pages and pages of these things, I finally found the one I was looking for - only to discover a little note I’d copied down in class, warning me that it was unsolvable due to a typographical error in the textbook.