@article{journals/jacm/Gottlob95,
  author = "Georg Gottlob",
  doi = "10.1145/201019.201031",
  journal = "{J. ACM}",
  number = "2",
  pages = "421-457",
  title = "NP Trees and Carnap's Modal Logic",
  volume = "42",
  year = "1995",
}