[OS X TeX] Baffled

juan tolosa juantolo at me.com
Thu Jan 30 22:53:41 EST 2020

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?


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$}\hfill
    \mkern -6mu \mathord \rightarrow$}

Any two distinct points $A$ and $B$ determine a unique line $\overleftrightarrow{AB}$.     


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