After running tlmgr update --all (I hadn't run it in a while) this code (from the dtx files all version of REVTeX 4 (version 4.2 is coming!)):

\expandafter\ifx\csname package@font\endcsname\@undefined\else
\expandafter\RequirePackage\expandafter{\csname package@font\endcsname}%


This is pdfTeX, Version 3.14159265-2.6-1.40.18 (TeX Live 2017) (preloaded   format=latex 2017.12.21)  22 DEC 2017 16:56
entering extended mode
restricted \write18 enabled.
%&-line parsing enabled.
LaTeX2e <2017-04-15>
Babel <3.15> and hyphenation patterns for 84 language(s) loaded.
! Missing \endcsname inserted.
<to be read again> 
l.3 \fi


This error doesn't happen if I run under older TeXLive installations and I didn't encounter until I ran the update. How can I figure out what broke?

Thanks, Mark

  • I just tried pdflatex revtex4.1.dtx in a texlive 2016 setup (the oldest I have to hand) and it gave this error Dec 22, 2017 at 22:42

1 Answer 1


This test

\expandafter\ifx\csname package@font\endcsname\@undefined

is never going to test true as \csname ljhgjghlj\encdname returns a csname \let to \relax so you always do

\RequirePackage\expandafter{\csname package@font\endcsname}%

even when it's not defined, in which case you get an error.

  • revtex4.dtx has the correct test against \relax but revtex4-1.dtx has this test against \@undefined which could never have worked unless you define \@undefined to be defined, which is weird. Dec 22, 2017 at 22:26
  • Can you imagine why the class doesn't use \@ifundefined{package@font}? I can't.
    – egreg
    Dec 22, 2017 at 22:45
  • Thanks David. I confess to being a bit too quick to post. This works under my much older (2014) TeXLive installation, but I guess when I was working on this under my initial 2017 install, I was only running tex and not latex to create the generated files and not create the formatted PDF docs. I’m just not sure at this point. Anyway, i will just remove the offending code or perhaps switch to the suggestion by egreg. Thanks!
    – Mark Doyle
    Dec 24, 2017 at 18:02
  • @MarkDoyle it could only work if you had some package that had erroneously defined \@ifundefined or that always ensured that \package@font was defined. The code fragment as posted in your question would always have given an error. Dec 24, 2017 at 18:04

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