Comment by tvali

https://tatsu.readthedocs.io/en/stable/ - this was my result to find lightweight syntax parsers. LLVM: in my experience, to play with little languages or ideas, such as additional tag, is so heavy-weight that it's as hard to learn as Isabelle Proof Assistant; large systems are interesting, but it's worth mentioning that 99% of the functionality could be often 1% of the API.