@inproceedings{ijcai1999p6,
  title     = {Preferred Arguments are Harder to Compute than Stable Extension},
  author    = {Dimopoulos, Yannis and Nebel, Bernhard and Toni, Francesca},
  booktitle = {Proceedings of the Sixteenth International Joint Conference on
               Artificial Intelligence, {IJCAI-99}},
  publisher = {International Joint Conferences on Artificial Intelligence},
  editor    = {Thomas Dean},
  year      = {1999},
  url       = {https://ijcai.org/proceedings/1999/6},
}