The classic Unix paper, "Typesetting Mathematics -- User's Guide" (2nd
ed.) remains even after 50 years an excellent resource for acquiring the
eqn language for preparing equations for *roff systems.

I wanted to retypeset this document with groff since its source is
available and the Seventh Edition Unix Programmer's Manual Volume 2
scans on the Web are caked with flyspecks and other unpleasantness.

So I did.

A further advantage of retypesetting it with groff is that we can
produce it in PDF, which has several advantages over PostScript and the
output devices used by Unix troff.

1. The text is searchable.

2. The text is navigable.  All this required was the addition of one
   macro call to the existing private `SC` macro defined by the
   document.  Et voilá--all the section headings are hyperlinked in a
   navigation pane (if your PDF reader offers one).

3. The text has metadata for the title and authors.  This was a matter
   of another two macro calls.

This exercise exposed several bugs in groff's ms macro package
implementation, the fixes for which are in groff 1.23.0.

More interesting was the exposure of a bug in AT&T troff: an equation
was missing from the Unix V7 C/A/T-typeset rendition of this manual.  It
is now restored (to the bottom of the left-hand column on page 4, when
rendered with groff 1.23.0).[1]

I made some very small changes to the source material, but none to
anything one might consider narrative; except for my addition of keep
macros (KS/KE), they are all commented and explained.  The most
important avoids lying about when the document was rendered.  I used
groff extensions unapologetically (but there wasn't much to do).

If you enjoy this document's transfiguration, please raise a glass to
the memory of Lorinda Cherry--who passed away in February 2022, mere
months before I undertook this project--and to her co-author, Brian
Kernighan, who is still with us as of this writing.

Many thanks to Deri James for his assistance with PDF features.

[1] You can obtain groff 1.23.0 from
    <https://ftpmirror.gnu.org/groff/groff-1.23.0.tar.gz>.