 The Boolean modal logic of Gargov and Passy 1990 and Gargov, Passy, and Tinchev 1987) follows a similar strategy. The difference is that, while PDL focuses on constructors for regular expressions (sequential composition, non-deterministic choice, finite iteration), Boolean modal logic focuses on constructors for the Boolean algebra over relations: complement \((\bdash)\), union \((\bcup)\) and intersection \((\bcap)\), together with a ‘global’ constant \((\boldsymbol{1})\). More precisely,