\begin{thebibliography}{Che76}

\bibitem[Che76]{Che76}
P.P. Chen.
\newblock The {Entity-Relationship} model --- toward a unified\ view of data.
\newblock {\em ACM Transactions on Database Systems}, 1(1):9--36, 1976.

\bibitem[PM98]{PM98}
A.~Poulovassilis and P.J. McBrien.
\newblock A general formal framework for schema transformation.
\newblock {\em Data and Knowledge Engineering}, 28(1):47--71, 1998.

\end{thebibliography}
