@article {Amati&96:IGPL,
   AUTHOR = {Amati, Gianni and Aiello, Luigia Carlucci and Gabbay, Dov and
             Pirri, Fiora},
   TITLE = {A structural property on modal frames characterizing default
             logic},
   JOURNAL = {J. IGPL},
   FJOURNAL = {Journal of the Interest Group in Pure and Applied Logics},
   VOLUME = {4},
   YEAR = {1996},
   NUMBER = {1},
   PAGES = {7--22},
   ISSN = {0945-9103},
}