Interdata_v6/usr/source/troff/eqn/read_me

Find at most related files.
including files from this version of Unix. 
The grammar for eqn is in e.g; don't change it
until you're reasonably cocky.
Semantic actions for most kinds of things are in e[1256].c;
input processing is in e3.c, general control in e4.c
(including the main routine), and
lexical stuff in elex.c. elex.c includes y.tab.c (from yaccing
e.g), so if you change the grammar you have to recompile
elex.c.

Make a new eqn with
	yacc e.g
	cc e*.c -ly -lS

The file test1 is a miniature test, small enough that
you can understand what it does by eyeballing the output.
When that works, use the file aip/browman for
bigger and more serious testing.
Both eqn and neqn have an undocumented option "-" which
turns on a fair amount of debugging information related
mainly to the heights of various objects.

Regrettably, neqn is somewhat different from eqn
internally, because it has not yet been converted
to use the new standard io.