@article{LW94,
  author = "Liskov, Barbara H. and Wing, Jeannette M.",
  title = "{A behavioral notion of subtyping}",
  journal = "ACM Transactions on Programming Languages and Systems",
  volume = "16",
  number = "6",
  month = "Nov.",
  year = "1994",
  url = "http://www.cs.berkeley.edu/~jcondit/pl-prelim/liskov94behavioral.ps",
}