Gianfranco Rossi Set-based Nondeterministic Declarative Programming in Singleton. 2002 76 Electr. Notes Theor. Comput. Sci. http://www.elsevier.com/gej-ng/31/29/23/126/23/show/Products/notes/index.htt#015 db/journals/entcs/entcs76.html#Rossi02