Irenes (many)have you ever thought wow, it would be nice to be able to embed TeX code in Markdown?
good news! so have many other people, and there are working implementations in Pandoc and mkdocs among other places
have you ever been slightly dissatisfied with a piece of software someone else wrote, and decided to write your own which does it the way you prefer, because after all it's small?
odds are a significant fraction of people reading this said yes, of course!
the roamerNot sure it counts, but I couldn't cope with the braces-heavy syntax of #LaTeX and wrote my own macro package in #PlainTeX. Have done books in it, all my lecture notes etc. Main design and core commands unchanged since 1997!
Syntax uses empty space and empty lines as delimiters. Eg, my source says
"\ch Introduction
In this book ..."
and TeX then typesets this as chapter 1 with title "Introduction" and "In this book ..." as first para.]