\documentclass{article}
\usepackage{a4}
\usepackage{url}
\usepackage{index}
\usepackage{natbib}
\makeindex\citeindextrue
\begin{document}
\title{``Natural''(Science)  Bibliographies}
\author{}
\date{}
\maketitle
\bibliographystyle{plainnat}
\newcommand{\zyva}
{\begin{center}
\begin{tabular}{p{.4\linewidth}@{\quad\ensuremath{\Rightarrow}\quad}p{.4\linewidth}}
\url+\citet{MPW92}+ & \citet{MPW92}\\
\url+\citet[chap.~2]{MPW92}+ & \citet[chap.~2]{MPW92}\\
\url+\citep{MPW92}+ & A~\citep{MPW92}\\
\url+\citep[chap.~2]{MPW92}+ & A~\citep[chap.~2]{MPW92}\\
\url+\citep[see][]{MPW92}+ & A~\citep[see][]{MPW92}\\
\url+\citep[see][chap.~2]{MPW92}+ & A~\citep[see][chap.~2]{MPW92}\\
\url+\citet*{MPW92}+ & \citet*{MPW92}\\
\url+\citep*{MPW92}+ & A~\citep*{MPW92}\\
\url+\citep[chap.~2]{MPW92,odersky:esop2000}+ & A~\citep[chap.~2]{MPW92,odersky:esop2000} \\
\url+\citet[chap.~2]{MPW92,odersky:esop2000}+ &\citet[chap.~2]{MPW92,odersky:esop2000} \\
\url+\citenum{MPW92}--\citenum{MPW92,odersky:esop2000}+ &
\citenum{MPW92}--\citenum{MPW92,odersky:esop2000}\\
\url+\citeauthor{MPW92}--\citeauthor{MPW92,odersky:esop2000}+ &
\citeauthor{MPW92}--\citeauthor{MPW92,odersky:esop2000}\\
\url+\citeauthor*{MPW92}--\citeauthor*{MPW92,odersky:esop2000}+ &
\citeauthor*{MPW92}--\citeauthor*{MPW92,odersky:esop2000}\\
\url+\citeyear{MPW92}--\citeyear{MPW92,odersky:esop2000}+ &
\citeyear{MPW92}--\citeyear{MPW92,odersky:esop2000}\\
\url+\citeyearpar{MPW92}--\citeyearpar{MPW92,odersky:esop2000}+ &
\citeyearpar{MPW92}--\citeyearpar{MPW92,odersky:esop2000}\\
\end{tabular}
\end{center}
\printindex}

\section{Default, from \texttt{.bst} file \texttt{plainnat}}
\zyva

%\citeindexfalse
\bibliography{natbib}
%\citeindextrue


\subsection{A few other comands}
\defcitealias{AppliedPi}{Applied~$\pi$}Now we
say~\citepalias{AppliedPi} for~\citep{AppliedPi}.
\begin{center}
\begin{tabular}{l@{\quad$\Rightarrow$\quad}l}
\url+\citetalias{AppliedPi}+ & \citetalias{AppliedPi}\\
\url+\citepalias{AppliedPi}+ & \citepalias{AppliedPi}\\
\url+\citepalias[See][Sec.~4]{AppliedPi}+ & \citepalias[See][Sec.~4]{AppliedPi}\\
\end{tabular}
\end{center}

\section{Option \texttt{authoryear}}
\setcitestyle{authoryear}\zyva

\section{Option \texttt{numbers}}
\setcitestyle{numbers}\zyva

\section{Option \texttt{super}}
\setcitestyle{super}\zyva

\end{document}
