[huntie]

I think it's usually to provide extra features. For example, Gerbil has a more advanced macro and module system than most Scheme implementations. It's also very easy to do because you're bascially given an AST for your new Scheme that you transform into the base Scheme.

Gerbil's docs have an example of this: https://github.com/vyzo/gerbil/blob/master/doc/tutorial/lang...

[kovrik]

Can these 'more advanced' features be implemented in, say, Racket/Guile/Chez?

[huntie]

I don't know why they couldn't be. I'm pretty sure Gerbil's macro/module systems are heavily inspired by Racket.