[pdftex] A bug in pdfTeX?

[Elio Fabri]

I am a novel subscriber, if the question I have is an old one, excuse
me. 

The following macro definition
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\def\tdot#1{#1\setbox\one=\hbox{$#1$}\oneh=\ht\one\onew=\wd\one
                    \setbox\two=\hbox{.\kern-1pt.\kern-1pt.}
                      \twoh=\ht\two\twow=\wd\two
         \advance\onew by -\twow \divide\onew by 2 \advance\twow by
\onew
         \advance\oneh by 1.5pt
         \kern-\twow\raise\oneh\box\two\kern\onew\relax}
------------------------------
gives an error message:
------------------------------
! Undefined control sequence.
\tdot #1->#1\setbox \one 
                         =\hbox {$#1$}\oneh =\ht \one \onew =\wd \one
\setbo...
l.17             \left(\tdot{\vq}
                                  \times \vn\right)\!\times \vn +
? q
OK, entering
\batchmode...                                                                         
! Missing number, treated as zero.
<to be read again>
                   =
\tdot #1->#1\setbox \one =
                          \hbox {$#1$}\oneh =\ht \one \onew =\wd \one
\setbo...
l.17             \left(\tdot{\vq}
                                  \times \vn\right)\!\times \vn +
A number should have been here; I inserted
`0'.                                                    
------------------------------
I am using pdftex for Linux, Version 3.14159-13d (Web2C 7.3.1).
The same macro works well in plain TeX.

Anyone can help? Thanks
-- 
Elio Fabri
Dip. di Fisica "Enrico Fermi" - Univ. di Pisa
Sez. Astronomia e Astrofisica
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