[texhax] Curious behavior of \cite

[Michael Barr]

Here is some curious behavior of \cite.  The definition of \@citex (the
macro that does the work), from latex.ltx is

\def\@citex[#1]#2{\leavevmode
  \let\@citea\@empty
  \@cite{\@for\@citeb:=#2\do
    {\@citea\def\@citea{,\penalty\@m\ }%
     \edef\@citeb{\expandafter\@firstofone\@citeb\@empty}%
     \if at filesw\immediate\write\@auxout{\string\citation{\@citeb}}\fi
     \@ifundefined{b@\@citeb}{\hbox{\reset at font\bfseries ?}%
       \G at refundefinedtrue
       \@latex at warning
         {Citation `\@citeb' on page \thepage \space undefined}}%
       {\@cite at ofmt{\csname b@\@citeb\endcsname}}}}{#1}}

It is the last line that actually inserts the citation and I will
concentrate on that.  First, \@cite at ofmt is actually a synonym of \hbox
and I really don't see why a synonym is needed, but maybe some change is
anticipated in the future.  At any rate, the effect of the \hbox is to
prevent the citation from breaking.  But since I generally use the
[Author (year)] style of citations, this causes all sorts of problems.
So I tried changing the last line to
       {\csname b@\@citeb\endcsname}}}{#1}}
This appeared to work fine, until I used \emph{et al.} in a citation.
To see the effect, run this file:

\documentclass{article}

\makeatletter
\def\@citex[#1]#2{\leavevmode
  \let\@citea\@empty
  \@cite{\@for\@citeb:=#2\do
    {\@citea\def\@citea{,\penalty\@m\ }%
     \edef\@citeb{\expandafter\@firstofone\@citeb\@empty}%
     \if at filesw\immediate\write\@auxout{\string\citation{\@citeb}}\fi
     \@ifundefined{b@\@citeb}{\hbox{\reset at font\bfseries ?}%
       \G at refundefinedtrue
       \@latex at warning
         {Citation `\@citeb' on page \thepage \space undefined}}%
       {\csname b@\@citeb\endcsname}}}{#1}}
\makeatother

\begin{document}

Here is an example of a citation that can cause
problems: \cite{A}

\begin{thebibliography}{xxx}

\bibitem[A \emph{et al.}]{A}

\end{thebibliography}
\end{document}

Of course, this happened to me with real citations, but I have reduced
it to the minimum.  What happens is that the citation has the form [A
\emph{et al.},] with an unwonted--not to mention unwanted--comma at the
end.  Eventually, I discovered that adding an extra pair of braces to
the last line:
       {{\csname b@\@citeb\endcsname}}}}{#1}}
cures the problem while still allowing breaks.  I cannot explain any of
this, but I did want to call the attention of texhaxxers to this
phenomenon.

Michael Barr