@inproceedings{Gibbons93:Upwards,
  title = "Upwards and Downwards Accumulations on Trees",
  author = "Jeremy Gibbons",
  year = "1993",
  booktitle = "Mathematics of Program Construction",
  editor = "R.~S. Bird and C.~C. Morgan and J.~C.~P. Woodcock",
  note = "A revised version appears in the Proceedings of the Massey Functional Programming Workshop, 1992",
  pages = "122--138",
  publisher = "Springer-Verlag",
  series = "Lecture Notes in Computer Science",
  url = "http://www.cs.ox.ac.uk/people/jeremy.gibbons/publications/accumulations.ps.gz",
  volume = "669",
}