What does your single file “productivity app” look like?

I would love to see what you managed to do in a single file, how it compares to other people in structure, functionality and technology. It doesn't have to be literally a "productivity app", but could also be how you, planningly or not, take notes, ideas or thoughts in a single file.

I got inspired by https://news.ycombinator.com/item?id=22276184 (My productivity app for the past 12 years has been a single .txt file).