[OS X TeX] Baffled

[G. M.-S.]

Try replacing
\m at th
with
\mathsurround=0pt
(which is the definition of \m at th).

Regards,

Guillermo

On Fri, 31 Jan 2020 at 04:54, juan tolosa via MacOSX-TeX <
macosx-tex at email.esm.psu.edu> wrote:

> I wanted to create a “double arrow overline” in plain TeX; I created the
> macro \overleftrightarrow by imitating what is done in the TeXbook in
> creating “overleftarrow” and “overrightarrow” After some tweaking, I got it
> to work. But here is the strange thing: by coincidence, I had the command
> “\input\miniltx.tex” at the beginning of the file, which I use to be able
> to draw pictures using the “graphicx” package. The problem is, if I remove
> this command, then the definition doesn’t work, the typesetter complains
> about the command “\m at th”
> I was blindly following the definitions in the TeXbook, my knowledge of
> the fine points in TeX is very (very!) limited. Still, I am baffled, since
> Knuth’s book deals with plain TeX, not with LaTeX.
>
> Can anyone illuminate me and let me know why is “miniltx” required for
> this definition to work?
>
> Thanks
> Juan
>
> Below is a sample file
>
> %
> \input miniltx.tex
> % \RequirePackage{graphicx}
>
> %
> %%%%%%%%%%%%%%%%%%
> %
> % DEFINE "over left right arrow". INSPIRATION:
> %. TeX book, definition of \overleftarrow on p. 359
> %.        plus definitions of \leftarrowfill and \rightarrowfill on p. 357
> % First need "left right arrow fill" defined
> \def\leftrightarrowfill{$\m at th \mathord\leftarrow \mkern-6mu
>     \cleaders\hbox{$\mkern-2mu \mathord- \mkern-2mu$}\hfill
>     \mkern -6mu \mathord \rightarrow$}
> %
> \def\overleftrightarrow#1{\vbox{\ialign{##\crcr
>       \leftrightarrowfill\crcr\noalign{\kern-1pt\nointerlineskip}
>       $\hfil\displaystyle{#1}\hfil$\crcr}}}
> %
> %________________________________________________________________
>
> Any two distinct points $A$ and $B$ determine a unique line
> $\overleftrightarrow{AB}$.
>
> \bye
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