Mauro Ferrari Camillo Fiorentini Guido Fiorino On the complexity of the disjunction property in intuitionistic and modal logics. 519-538 2005 6 ACM Trans. Comput. Log. 3 http://doi.acm.org/10.1145/1071596.1071598 db/journals/tocl/tocl6.html#FerrariFF05