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Nonmonotoni Reasoning Joeri Engelfriet1, V. Wiktor Marek2, Jan Treur1, and Miros law Trusz zy nski2 1Free University Amsterdam Department of Mathemati s and Computer S ien e Arti ial Intelligen e Group De Boelelaan 1081a, 1081 HV Amsterdam The Netherlands fjoeri, treurg s.vu.nl Phone: +31 20 4447756, +31 20 4447763, Fax: +31 20 4447653. 2Department of Computer S ien e University of Kentu ky Lexington, KY 40506-0046 USA fmarek, mirekg s.engr.uky.edu Phone: +(606) 257-3961, Fax: +(606) 323-1971 Short Title: Default Logi and Spe i ation of Reasoning