[groff] Design and Implementation of *roff

[Yves Cloutier]

Hello,

I'm in search of any documentation that provides insight into the design
and implementation of a *roff.

I know source code exists for a few implementations like Plan 9, Heirloom,
Groff and Neatroff.

However what I'm in search of is something a bit more high-level than
source code. Rather some design/implementation documentation that explains
an *approach* to implementing a *roff.

Would anyone know if anything like this exists?

My google searches how not come up with anything, and was just wondering if
someone somewhere might have such documentation lying around in their
archives.

regards,

yc