[Groff] Is _M considered an internal register of -me?

[James J. Ramsey]

I wanted to redefine the ".$c" macro in -me to print
chapter headings the way I wanted them printed.
However, I noticed that by default ".$c" used the _M
register, which is used by the ".++" macro to indicate
whether one is in the prefatory matter section, the
chapter section, appendix section, etc. I was thinking
of using the _M register in my redefined ".$c" macro
for the same purpose, but I wasn't sure that _M was a
"safe" register to use, that is, a register that I can
count on existing from version to version of -me. I
have not found any reference to register _M in the -ME
Reference Manual, nor in the man page. So far as I can
tell, it seems to be solely for internal use, yet it
is used in macros that are intended to be redefined.

Can I count on the _M register to remain in at least
the groff version of the -me macros, Or should I write
a ".$c" macro under the assumption that _M may not
always be available?


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