@inproceedings{LambekSheaf,
  author =       "Joachim Lambek",
  title =        "On the Sheaf of Possible Worlds",
  year =         1989,
  title =        "Categorical Topology and its relation to Analysis,
                  Algebra and Combinatorics",
  pages =        "36--53",
  publisher =    "World Scientific Publishing"
}