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\documentclass[12pt]{article}
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\begin{document}

\begin{enumerate}
\item
\verb|\cite{goossens:companion}| ---
\cite{goossens:companion}
\item
\verb|\cite{goossens:companion,hagen:metafun}| ---
\cite{goossens:companion,hagen:metafun}
\item
\verb|\cite[page 99ff]{goossens:companion}| ---
\cite[page 99ff]{goossens:companion}
\item
\verb|\cite[page 99ff]{goossens:companion,hagen:metafun}| ---
\cite[page 99ff]{goossens:companion,hagen:metafun}
\end{enumerate}

\bibliography{natbib}

\end{document}
