@article{journals/jacm/GottlobKS06,
  title = "Conjunctive queries over trees",
  author = "Georg Gottlob and Christoph Koch and Klaus U. Schulz",
  year = "2006",
  journal = "J. {ACM}",
  number = "2",
  pages = "238-272",
  volume = "53",
  doi = "10.1145/1131342.1131345",
}