@inproceedings{CaGP09,
  title = "Tractable Query Answering over Conceptual Schemata",
  author = "Cal\`{\i}, Andrea and Gottlob, Georg and Pieris, Andreas",
  year = "2009",
  booktitle = "Proceeding of the 28th International Conference on Conceptual Modeling (ER)",
  pages = "175--190",
}