@InProceedings{MEDLAR94,
       author     =      " Amati,  G. and  Carlucci Aiello,  L. and  Gabbay, D. and Pirri, F.",  
       title  =   "Provability  in non monotonic modal logic by balloons and Christmas trees",
       booktitle  =   " MEDLAR, An Esprit Basic Research Project",
       publisher =  "Imperial College of Science, Technology and Medicine",
       address  =  " London, UK",
       month =jul,
       year =    "1994", 
       notes = "Deriverable II 5.1, Project  MEDLAR II  Esprit Basic Research Project 6471",
       pages = "253--282",
 }