A correction of: H. Prakken, An abstract framework for argumentation with structured arguments. Argument and Computation 1 (2010): 93-124. [PDF] Definition 6.8 of a well-formed argumentation theory is incorrect, since it allows for counterexamples to some results. The correct definition is: An argumentation theory is well-formed iff: if \phi is a contrary of \psi then \psi is not in Kn and \psi is not the consequent of a strict rule. The correct definition has meanwhile been published in S. Modgil & H. Prakken, Revisiting preferences and argumentation. Proceedings of the 22nd International Joint Conference on Artificial Intelligence (IJCAI 2011), 1021-1026. Barcelona (Spain), 2011. [PDF] S.J. Modgil & H. Prakken, A general account of argumentation with preferences. Artificial Intelligence 2013, in press. [PDF]. (Version at AI Journal homepage).

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