Iterators in Scheme?

[Niols]

Hello,

I am trying to dive a bit into the way LilyPond works, and I was 
wondering about iterators. The documentation contains a page named 
“iterator tutorial” [1] but this is only a placeholder for a future 
tutorial. (I very much understand that it is painful to write such 
tutorials and I am not here to complain of anything!)

[1] 
https://lilypond.org/doc/v2.20/Documentation/contributor/iterator-tutorial

The only thing the page actually says is “Iterators are routines written 
in C++ that process music expressions and sent the music events to the 
appropriate engravers and/or performers.”

Must I understand that it is not possible to write iterators in Scheme 
and that they have to be written in C++?

Must I then understand that, if I wanted to write iterators for a 
personal use, I would have to recompile LilyPond myself?

If it is actually possible to write an iterator in Scheme, would anyone 
have an example to show me or a link to give me?

Thank you so much in advance.

Best,
— Niols