\begin{thebibliography}{1}

\bibitem{requirements1}
R.~T. Yeh and P.~Zave, {\em Specifying Software Requirements}.
\newblock Proceedings of the IEEE, vol. 68~ed., 1980.

\bibitem{mod_struct_anal}
E.~Yourdon, {\em Modern Structure Analysis}.
\newblock Yourdon Press, 1st~ed., 1989.

\bibitem{aufwandschaetzung}
S.~Vollmann, {\em Aufwandschaetzung im Software Engineering}.
\newblock IWT, 1st~ed., 1990.

\bibitem{use_case1}
M.~Fowler, ``Use cases and abuse cases,'' {\em www.softwarefuture.com}, June
  1998.

\bibitem{use_case2}
E.~Kenworthy, ``Use case modelling - capturing user requirements,'' {\em
  www.zoo.co.uk}, December 1997.

\end{thebibliography}
