Incomplete \iffalse when using syntax package with a citation

Question

For reasons unknown to me the following minimised latex code produces a

(/opt/local/share/texmf-texlive/tex/latex/mdwtools/syntax.sty) (./text.aux)) ! Incomplete \iffalse; all text was ignored after line 12.

error by pdflatex. Its derived from my document where the trouble started when I tried to add the syntax package:

\documentclass[a4paper,fontsize=12pt,parskip=half,toc=listofnumbered,toc=bib]{scrreprt}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}


\usepackage{syntax}

\begin{document}

\begin{quotation}
As a rule of thumb, if you can tell which computer you are using, you are not using a distributed system.
\end{quotation} from \cite{DOS_Tanenbaum}

\end{document}

Is seems it somehow interferes with bibtex, since when I remove the \cite it generates a pdf.

Answer 1

The simplest solution is to do \usepackage[nounderscore]{syntax}. However, you can fix the usage in labels or citations.

\documentclass{article}

\usepackage{syntax}

\makeatletter
\def\@uscore.{%
  \ifmmode
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi
  \sb
  {\@usc@re}%
}
\def\@usc@re{%
  \ifincsname
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi
  {\string_}%
  {\textunderscore\@ifnextchar_{}{\usc@builtindischyphen}}%
}
\if@uscore
  \AtBeginDocument{%
    \catcode`\_\active%
    \begingroup%
    \lccode`\~`\_%
    \lowercase{\endgroup\protected\def~{\@uscore.}}%
  }
\fi


\begin{document}

This is an underscore ``_''; also \cite{DOS_Tanenbaum} and $a_1$

\end{document}