@techreport{TractableConstraints,
  title = "Tractable constraints closed under a binary operation",
  author = "Andrei Bulatov and Peter Jeavons",
  year = "2000",
  institution = "Oxford University Computing Laboratory",
  number = "PRG-TR-12-00",
  pages = "27",
  url = "http://www.cs.ox.ac.uk/techreports/oucl/TR-12-00.ps.gz",
}