ptimality in Abstract Argumentation Iyad Rahwan 1 Faculty of Informatics, British University in Dubai P.O.Box 502216, Dubai, UAE 2 (Fellow) School of Informatics, University of Edinburgh Edinburgh, EH8 9LE, UK Kate Larson Cheriton School of Computer Science University of Waterloo 200 University Avenue West Waterloo, ON, N2L 3G1, Canada