@phdthesis{Gibbons91:Algebras,
  title = "Algebras for Tree Algorithms",
  author = "Jeremy Gibbons",
  year = "1991",
  note = "Available as Technical Monograph PRG-94. ISBN 0-902928-72-4",
  school = "Programming Research Group, Oxford University",
  url = "http://www.cs.ox.ac.uk/files/3422/PRG94.pdf",
}