Order:
  1.  12
    Formalizing nonmonotonic reasoning systems.David W. Etherington - 1987 - Artificial Intelligence 31 (1):41-85.
  2.  5
    Nonmonotonicity and the scope of reasoning.David W. Etherington, Sarit Kraus & Donald Perlis - 1991 - Artificial Intelligence 52 (3):221-261.