[OS X TeX] Baffled

[juan tolosa via MacOSX-TeX]

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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