Christoph Beierle Reinhold Kloos Gregor Meyer A Pragmatic Type Concept for Prolog Supporting Polymorphism, Subtyping, and Meta-Programming. 1999 http://www.elsevier.nl/locate/entcs/volume30.html 30 Electr. Notes Theor. Comput. Sci. 1 db/journals/tcs/entcs30.html#BeierleKM99