@article{AC93,
  author = "Amadio, Roberto M. and Cardelli, Luca",
  title = "{Subtyping recursive types}",
  journal = "ACM Transactions on Programming Languages and Systems",
  volume = "15",
  number = "4",
  month = "Sep.",
  year = "1993",
  url = "http://www.cs.berkeley.edu/~jcondit/pl-prelim/amadio93subtyping.ps",
}