@Techreport{BagnaraHZ04TRa,
  Author = "R. Bagnara and P. M. Hill and E. Zaffanella",
  Title = "Widening Operators for Powerset Domains",
  Number = 349,
  Type = "Quaderno",
  Institution = "Dipartimento di Matematica, Universit\`a di Parma, Italy",
  Year = 2004,
  Note = "Available at \url{http://www.cs.unipr.it/Publications/}",
}