\@undefined appears several times in the pgf/tikz source-code. For instance, in the file
<pgf/tikz install dir>/utilities/pgfkeys.code.tex it appears twice:
\expandafter\ifx\csname#1\endcsname\relax% \else% \expandafter\ifx\csname#1\endcsname\@undefined% \else\pgfkeys@csname@testtrue% \fi% \fi
but it appears in other files as well.
Unfortunately, I couldn't find where
\@undefined is defined in the pgf/tikz source-code. It is possible that I didn't search properly, because the letters
@ are tricky.
- What does
\@undefinedmean in the context of the pgf/tikz source-code?
- Is it defined anywhere in the pgf/tikz source-code, or is it a known TeX control-sequence (or some extension of TeX, e.g. e-TeX) that I'm simply not familiar with?
I've considered the possibility that
\@undefined is simply some control-sequence that is assumed to be undefined, and its purpose is to help test whether other control-sequences are defined using the idiom
However, if this is indeed the intended use, then I don't understand the first example above, since the expression
will always evaluate to
false: this is obvious if the control-sequence
\csname#1\endcsname is defined, and if it isn't defined, then the
\csname...\endcsname construct will define it to be
\relax (this is just how the
\csname...\endcsname construct works -- see exercise 7.7 on p. 40 of the TeXbook, 20th printing 1991).