@techreport{Zivny07:expressiveTR,
  title = "The expressive power of valued constraints: hierarchies and collapses",
  author = "David A. Cohen and Peter G. Jeavons and Stanislav \v{Z}ivn\'y",
  year = "2007",
  address = "Oxford, UK",
  institution = "Computing Laboratory, University of Oxford",
  month = "April",
  number = "RR-07-03",
  url = "http://web.comlab.ox.ac.uk/oucl/publications/tr/RR-07-03.html",
}