I have no plans to work on this for the reasons in comment #1 but someone else is welcome to take it; setting to postponed.

From ad1a4520a3aafc05db1988f285371395fa0ef811 Mon Sep 17 00:00:00 2001
From: Bjarni Ingi Gislason <bjarniig@rhi.hi.is>
Date: Wed, 24 Nov 2021 22:58:09 +0000
Subject: [PATCH] src/preproc/grn/grn.1.man: use a unique character for the
 tabulator in a table

tbl:./src/preproc/grn/grn.1.man:723: error: excess data entry 'g' discarded
tbl:./src/preproc/grn/grn.1.man:723: error: excess data entry 'troff\f[])' discarded
tbl:./src/preproc/grn/grn.1.man:724: error: excess data entry 'g' discarded
tbl:./src/preproc/grn/grn.1.man:724: error: excess data entry 'troff\f[])' discarded
tbl:./src/preproc/grn/grn.1.man:725: error: excess data entry 'g' discarded
tbl:./src/preproc/grn/grn.1.man:725: error: excess data entry 'troff\f[])' discarded
tbl:./src/preproc/grn/grn.1.man:726: error: excess data entry 'g' discarded
tbl:./src/preproc/grn/grn.1.man:726: error: excess data entry 'troff\f[])' discarded

  "@" is both used as a tabulator character and as a part of a string
constant "@g@" in a table.

  Change the tabulator character in the table from "@" to "#".

Signed-off-by: Bjarni Ingi Gislason <bjarniig@rhi.hi.is>
---
 src/preproc/grn/grn.1.man     | 10 +++++-----
 1 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/src/preproc/grn/grn.1.man b/src/preproc/grn/grn.1.man
index c1aa3beb3..02fdbeabc 100644
--- a/src/preproc/grn/grn.1.man
+++ b/src/preproc/grn/grn.1.man
@@ -718,12 +718,12 @@ the brush selects a font as follows.
 .
 .IP
 .TS
-center, tab(@);
+center, tab(#);
 nB l.
-1@roman (R font in \f[I]@g@troff\f[])
-2@italics (I font in \f[I]@g@troff\f[])
-3@bold (B font in \f[I]@g@troff\f[])
-4@special (S font in \f[I]@g@troff\f[])
+1#roman (R font in \f[I]@g@troff\f[])
+2#italics (I font in \f[I]@g@troff\f[])
+3#bold (B font in \f[I]@g@troff\f[])
+4#special (S font in \f[I]@g@troff\f[])
 .TE
 .
 .
--
2.33.0