There were enough interesting suggestions I created a new release of prep(1) that incorporates them all (except for a modern modularized rewrite of the expression parser, I just have not had time (I started it off with some OLD code)) in
Release v8.1.1 of prep(1) - a Fortran preprocessor · urbanjost/prep · GitHub
Thanks for the feedback. Suggestions still welcome! The processing of extended markdown and tex files; and the --debug mode along with the $HELP cribsheet for interactive tests and “$BLOCK SET” are my favorites so far.