[troff]: Implement new `.nn` register

...to report the remaining count of lines to have their numbering
suppressed.

* src/roff/troff/env.h (class environment): Declare new member function
  `get_no_number_count()`.

* src/roff/troff/env.cpp (get_no_number_count): Define new member
  function, returning value of member variable `no_number_count`.

  (init_env_requests): Define new ".nn" register and attach it to
  `get_no_number_count()`.

* doc/groff.texi (Miscellaneous): Document it.  Include example of use
  to determine whether current output line will be numbered.  Also
  clarify meaning of register; `.nn` is not decremented except when
  output line numbering is enabled.

* man/groff.7.man (Read-only registers): Document it.

* src/roff/groff/tests/dot-nn_register_works.sh: Test it.
* src/roff/groff/groff.am (groff_TESTS): Run test.

* NEWS: Add item.

See <https://lists.gnu.org/archive/html/groff/2022-11/msg00153.html>.

1 files changed, 9 insertions, 6 deletions

diff --git a/NEWS b/NEWS
index 356856db3..1a2c75fd6 100644
--- a/NEWS
+++ b/NEWS
@@ -22,12 +22,15 @@ o A new read-only register `.cp` is implemented.  Within a `do` request,
   groff_diff(7) or the groff Texinfo manual for rationale, use case, and
   example.
 
-o A new read-only register `.nm` is implemented.  It reports the
-  enablement status of output line numbering (caused by the `nm`
-  request) irrespective of the temporary suspension of numbering with
-  the `nn` request.  This register was introduced because there was no
-  way to introspect its state, tbl(1) needs to be able to do so, and the
-  writable line number register `ln` is not a reliable proxy for it.
+o New read-only registers `.nm` and `.nn` are implemented.  `.nm` is of
+  Boolean sense, reporting the enablement status of output line
+  numbering (caused by the `nm` request) irrespective of the temporary
+  suspension of numbering with the `nn` request.  `.nn` holds the count
+  of numbered output lines still to have that numbering suppressed.
+  These registers were introduced because there was no way for the
+  formatter (and thus a document) to introspect their state, tbl(1)
+  needs to be able to do so, and the writable line number register `ln`
+  is not a reliable proxy for this information.
 
 o Type size escape sequences of the form "\sNN", where NN is in the
   range 10-39, are now recognized only in compatibility mode ("groff