[Groff] .if !dTS - GNU extension?

[John Gardner]

Hey everybody,

I was digging through Groff's source code when I happened across this line
<http://git.savannah.gnu.org/cgit/groff.git/tree/src/preproc/tbl/main.cpp#n1600>
:

printf(".if !\\n(.g .ab GNU tbl requires GNU troff.\n"
>        ".if !dTS .ds TS\n"
>        ".if !dTE .ds TE\n"


Perhaps I'm missing something, but I wasn't aware the .if request supported
runtime checking of macro definitions by using !d. There's nothing in
the classical
Troff manual <http://www.troff.org/54.pdf> about this, which simply lists
the basic boolean, numeric and string comparisons (page 24).

This left me wondering if it's a Groff extension, but I couldn't find any
mention of it in *groff_diff*. Or is there something else I'm missing?

Many thanks for any elucidation!
- John