Patrick Blackburn Thomas Bolander Torben Braüner Valeria de Paiva Jørgen Villadsen 1-2 2007 174 Electr. Notes Theor. Comput. Sci. 6 http://dx.doi.org/10.1016/j.entcs.2006.11.021 db/journals/entcs/entcs174.html#BlackburnBBPV07