% de naeste seks makroer anvendes internt
% de kan uden problemer fjernes i hele filen
\let\START\relax
\let\STOP\relax
\let\SPECEXTSTART\relax
\let\SPECEXTSTOP\relax
\let\DCBEGIN\relax
\let\DCEND\relax
 \DCBEGIN
\documentclass[a4paper]{article}
 \DCEND

 \usepackage [latin1]{inputenc}
 \usepackage [T1]{fontenc}
 \usepackage [danish]{babel}
 \usepackage {felineekssupp}
 \usepackage {kpfonts}
 \usepackage [scaled=0.8]{beramono}
 \usepackage [scaled=0.85]{berasans}

% al anvendelse af geometry i eksemplerne boer man slette,
% dette anvendes mht. at stoerrelsen skal passe til brug i bogen
\usepackage{geometry}
\geometry{textwidth=412.0pt}
% vil ikke have sidetal med, fjern selv
\pagestyle{empty}
% forfatteren snyder maaske, fjern kode fra HER -->

% <-- til HER.
 \START
\usepackage{tikz}
\usetikzlibrary{matrix}
 \STOP

\begin{document}
\[
\begin{tikzpicture}
  \matrix (M)
  [matrix of math nodes, row sep=2em,column sep=2em ]
  {
    A & B \\  C & D \\ % sidste \\ er VIGTIG
  };
  \begin{scope}[execute at begin node=$\scriptstyle, execute at end node=$]
    \draw[->] (M-1-1) -- node[above] {\alpha}           (M-1-2);
    \draw[->] (M-1-1) --                                (M-2-1);
    \draw[->] (M-1-2) -- node[left] {\beta}             (M-2-2);
    \path     (M-2-1) -- node {\displaystyle\subseteq}  (M-2-2);
  \end{scope}
\end{tikzpicture}
\]
\end{document}