1. Test-free Propositional Dynamic Logic is Strictly weaker than PDL;Berman,1977
2. Expressiveness hierarchy for PDL with rich tests;Berman,1978
3. Propositional Dynamic Logic of programs;Fischer;J. Comput. System Sci.,1979
4. The Mathematical Theory of Context-Free Languages;Ginsburg,1966
5. G. Peterson, The power of tests in Propositional Dynamic Logic, Technical Report #47, University of Rochester, Rochester, NY.