@inproceedings{Gibbons98:Polytypic,
  title = "Polytypic Downwards Accumulations",
  author = "Jeremy Gibbons",
  year = "1998",
  address = "Marstrand, Sweden",
  booktitle = "Proceedings of Mathematics of Program Construction",
  editor = "Johan Jeuring",
  month = "jun",
  pages = "207-233",
  publisher = "Springer-Verlag",
  series = "Lecture Notes in Computer Science",
  url = "http://www.cs.ox.ac.uk/people/jeremy.gibbons/publications/polyda.ps.gz",
  volume = "1422",
}