C. Schulz, J. Alexandersson, and T. Becker. Proceedings of the Eight International Conference on Computational Semantics, Tilburg, The Netherlands, page 320-325. (2009)
R. Pieraccini, and J. Huerta. Recent Trends in Discourse and Dialogue, volume 39 of Text, Speech and Language Technology, chapter 1, Springer, Dordrecht, (2008)