Sabine Broda Luís Damas Studying provability in implicational intuitionistic logic the formula tree approach. 2002 67 Electr. Notes Theor. Comput. Sci. http://dx.doi.org/10.1016/S1571-0661(04)80545-0 db/journals/entcs/entcs67.html#BrodaD02 131-147