Ask HN: Is there an easy example of a Lisp pretty-printer?

I'm working on a pretty printer for my own (toy) Lisp and it's harder than I thought. I've been looking for easy to understand example code, a pretty printer in Scheme or Lisp that I could adapt and extend but so far have only found academic overview papers from the 80s. It doesn't help that most of the stuff is for CommonLisp and uses format. I don't have any special string formatting directives and my Lisp is rather different from CL, more akin to XLisp.

Do any gray-bearded Lisp programmers remember some old Lisp or Scheme example code that I could study?