I had a math question that had some free parameters, making it a sort of question machine, giving one question for each setting of the parameters. It was hard to keep track of my thoughts and partial answers about the question-family, so I made a multi-dimensional spreadsheet in vim, so I could see at a glance which questions I'd answered or not.

More details in this blog post: https://tsvibt.blogspot.com/2022/06/multisheets-multi-dimens...