[bug #62692] [eqn] want a way to recover "set" parameteric defaults.

[G. Branden Robinson]

URL:
  <https://savannah.gnu.org/bugs/?62692>

                 Summary: [eqn] want a way to recover "set" parameteric
defaults.
                 Project: GNU troff
               Submitter: gbranden
               Submitted: Fri 01 Jul 2022 05:37:15 AM UTC
                Category: Preprocessor eqn
                Severity: 1 - Wish
              Item Group: Feature change
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None


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Follow-up Comments:


-------------------------------------------------------
Date: Fri 01 Jul 2022 05:37:15 AM UTC By: G. Branden Robinson <gbranden>
See the griping comment in the following diff to Kernighan & Cherry's
"Typesetting Mathematics--User's Guide" (2nd ed.) below.


@@ -428,11 +432,18 @@
 right ]
 .P2
 will make
+.\" Cheat #1: AT&T eqn stacked piles more tightly than GNU eqn.
 .EQ
+set baseline_sep 1v
 A ~=~ left [
 pile { a above b above c } ~~ pile { x above y above z }
 right ]
 .EN
+.\" Reset equation baseline separation to the default, which is not
+.\" documented anywhere and has no syntactical access.  :-/
+.EQ
+set baseline_sep 140
+.EN
 The elements of the pile (there can be as many as you want)
 are centered one above another, at the right height for
 most purposes.


These "set" parameters are GNU extensions.  As far as I can see there is no
way within the eqn language to copy their values (say into an eqn macro), and
no way to access the compiled-in default once it is changed.  This appears to
be because they are stored as (mutable) global symbols in the executable.  

https://git.savannah.gnu.org/cgit/groff.git/tree/src/preproc/eqn/box.cpp

I think all of these parameters are numeric.  If that bears out then I suggest
supporting a special syntax, like


set baseline_sep @


or some other punctuation symbol to recover the default.







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