@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/}", }