Conservative extensions in modal logic

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\begin{bibtex} @inproceedings{DBLP:conf/aiml/GhilardiLWZ06,

 author    = {Silvio Ghilardi and
              Carsten Lutz and
              Frank Wolter and
              Michael Zakharyaschev},
 title     = {Conservative extensions in modal logic},
 booktitle = {Advances in Modal Logic},
 year      = {2006},
 pages     = {187-207},
 ee        = {http://www.aiml.net/volumes/volume6/Ghilardi-Lutz-Wolter-Zakharyaschev.ps},
 crossref  = {DBLP:conf/aiml/2006},
 bibsource = {DBLP, http://dblp.uni-trier.de}

}

\end{bibtex}