The Free On-line Dictionary of Computing (30 December 2018):

Portable Scheme Debugger

   (PSD) A package for source code debugging of R4RS-compliant
   Scheme under GNU Emacs by Kellom ?ki Pertti
   .  Version 1.1.  Distributed under GNU GPL.
   It works with scm, Elk and Scheme->C.

   (ftp://ftp.cs.tut.fi/pub/src/languages/schemes/psd.tar.Z).

   (1992-10-08)