By way of a thank you to the group...

I was looking in to the existence of m4 on OS-9 (the 6809 OS, not the Macintosh one).

It's not there, but a google-daisy-chain let me to m4's father,
GPM (general purpose macrogenerator) by Christopher Strachey, circa 1965.

There's a paragraph on the use of this lovely new toy, which may
chime with 6502 programmers in general, and 2600 programmers in particular:

It has been our experience that the GPM, while a
very powerful tool in the hands of a ruthless programmer,
is something of a trap for the sophisticated one. it
contains in itself all the undesirable features of every
possible machine code -- in the sense of inviting endless
tricks and time-wasting through fascinating exercises in
ingenuity -- without any of the irritating ad-hoc feature
of real machines. It can also be almost impenetrably
opaque, and even very experienced programmers indeed
tend to spend hours simulating its actions when one of
their macro definitions goes wrong. Furthermore it is
remarkably good at using up machine time--fortunately
the programs written for it are usually rather short.


link to full paper (8 Mb PDF)

BugBear