@article{LSAS77,
  author = "Liskov, Barbara and Snyder, Alan and Atkinson, Russell and Schaffert, Craig",
  title = "{Abstraction mechanisms in CLU}",
  journal = "Communications of the ACM",
  volume = "20",
  number = "8",
  month = "Aug.",
  year = "1977",
  url = "http://www.cs.berkeley.edu/~jcondit/pl-prelim/liskov77clu.pdf",
}