@article{Gibbons*2001:When,
  title = "When is a Function a Fold or an Unfold?",
  author = "Jeremy Gibbons and Graham Hutton and Thorsten Altenkirch",
  year = "2001",
  journal = "Electronic Notes in Theoretical Computer Science",
  month = "apr",
  note = "Proceedings of Coalgebraic Methods in Computer Science",
  number = "1",
  url = "http://www.cs.ox.ac.uk/people/jeremy.gibbons/publications/kernels.ps.gz",
  volume = "44",
}