@Inproceedings{Ghilardi-Lutz-Wolter-Zakharyaschev-06,
  author    = {Silvio Ghilardi and Carsten Lutz and Frank Wolter and Michael Zakharyaschev},
  title     = {Conservative extensions in modal logic},
  booktitle = {Proceedings of AiML 6: the 6th Conference on Advances in Modal Logic, 
               Noosa, Queensland, Australia, 25--28 September 2006},
  editor    = {Guido Governatori and Ian M. Hodkinson and Yde Venema},
  year      = {2006},
  pages     = {187--207},
}