Any explanation for the following problem? The native LaTeX compiler has no problem with examples 1 and 2. On example 3 it complains: ‘Argument of \xhalign has an extra }’. When compiled in isolation, examples 2 and 3 produce the same tracing until \xhalign is invoked.
\tracingcommands=2
\tracingmacros=2
\documentclass{article}
\begin{document}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\catcode‘\@=11
\def\eqnarray{%
\stepcounter{equation}%
\def\@currentlabel{\p@equation\theequation}%
\global\@eqnswtrue
\m@th
\global\@eqcnt\z@
\tabskip\@centering
\let\\\@eqncr
$$\everycr{}\xhalign \cr
}
\def\xhalign#1\cr{%
\halign to\displaywidth\bgroup
\hskip\@centering$\displaystyle\tabskip\z@skip{##}$\@eqnsel
&\global\@eqcnt\@ne\hskip \tw@\arraycolsep \hfil${##}$\hfil
&\global\@eqcnt\tw@ \hskip \tw@\arraycolsep
$\displaystyle{##}$\hfil\tabskip\@centering
&\global\@eqcnt\thr@@ \hb@xt@\z@\bgroup\hss##\egroup
\tabskip\z@skip
\cr
}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\vbox\bgroup %% example 1 %%
\begin{eqnarray} x \end{eqnarray}%
\egroup
\begin{tabular}{c} %% example 2 %%
\vbox{%
\begin{eqnarray} x \end{eqnarray}%
}%
\end{tabular}
\begin{tabular}{c} %% example 3 %%
\vbox\bgroup
\begin{eqnarray} x \end{eqnarray}%
\egroup
\end{tabular}
\end{document}

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