[pdftex] A bug in pdfTeX?

[Robin Fairbairns]

> \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 

so: \one is not defined

> The same macro works well in plain TeX.

in the context of some other macro that defines \one -- it's not
defined in plain tex "out of the box".

this is *very* simple error diagnosis.  there's some pompous words (by
me) on the topic in

  http://www.tex.ac.uk/cgi-bin/texfaq2html?label=erroradvice

and succeeding answers