This made me think of Racket's syntax objects, which are also more than simple lists. They contain a lot of information like source and position, but can also c

This made me think of Racket's syntax objects, which are also more than simple lists. They contain a lot of information like source and position, but can also contains arbitrary properties (like a map, in a sense). See https://docs.racket-lang.org/reference/stxprops.html https://docs.racket-lang.org/reference/stxprops.html