\begin{thebibliography}{1}

\bibitem{this}
Formelt bevis for den kommutative lov for addition i peano-aritmetik.
\newblock
  \url{http://www.diku.dk/~grue/logiweb/20050502/home/hyldahl/peanorapport/fix%
ed/}.

\bibitem{logiweb}
Logiweb.
\newblock \url{http://yoa.dk}.

\bibitem{basepage}
Logiweb base page.
\newblock
  \url{http://www.diku.dk/~grue/logiweb/20050502/home/grue/base/latest/}.

\bibitem{peanopage}
Peano.
\newblock
  \url{http://www.diku.dk/~grue/logiweb/20050502/home/grue/peano/latest/}.

\bibitem{mendelson}
E.~Mendelson.
\newblock {\em Introduction to Mathematical Logic, fourth edition}.
\newblock Chapman \& Hall, 1997.

\end{thebibliography}
