Conflict between catchfilebetweentags and a kind of arrow from xypic: argument of \next@ has an extra }

Question

I'm using the solution from https://tex.stackexchange.com/a/4952/1403 to input a part of file.tex into another file.

But I'm getting this error:

at the line \ExecuteMetaData[file.tex]{tag}.

Here's a MWE. The file to be recalled is:

%<*tag>

\begin{equation}  \xymatrix{A\ar@(ul,ur)^\Sigma }\end{equation}

%</tag>

Here is the main file:

\documentclass[a4paper,12pt]{amsbook} 
\usepackage{catchfilebetweentags}
\usepackage{xypic}
\begin{document}

\ExecuteMetaData[file.tex]{tag}

\end{document}

The xypic code being recalled does work if I don't recall it, but rather write the line directly on the main file.

Answer 1

catchfilebetweentags reads the file with the catcode of the @ set to letter. This breaks the \ar@ command of xypic as it espect the catcode of @ to be "other".

The problem can be demonstrated without external file (see below). I don't see a good way to avoid the \makeatletter in catchfilebetweentags, it is quite hardcoded and removing it breaks other code. What you can do is to define a command that you use instead of the \ar@ in the xypic:

\documentclass[a4paper,12pt]{amsbook}
\usepackage{catchfilebetweentags}
\usepackage{xypic}

\newcommand\myarat{\ar@}

\begin{document}

\makeatletter %done by catchfilebetweentags 
%<*tag>

%\begin{equation}  \xymatrix{A\ar@(ul,ur)^\Sigma }\end{equation} %error

\begin{equation}   \xymatrix{A\myarat(ul,ur)^\Sigma }\end{equation}

%</tag>

\end{document}

If you want to keep the \ar@ you can try this:

\documentclass[a4paper,12pt]{amsbook}
\usepackage{catchfilebetweentags}
\usepackage{xypic}


\begin{document}
\CatchFileBetweenTags{\mytemp}{file}{tag}
\scantokens\expandafter{\mytemp}


\end{document}