1. B.F. Chellas,Modal Logic (Cambridge University Press, 1980).
2. R. Fagin and J.Y. Halpern, Belief, awareness, and limited reasoning, Art. Int. 34 (1988) 39?76.
3. R. Fagin and J.Y. Halpern, Reasoning about knowledge and probability: preliminary report, in:Proc. 2nd Conf. on Theoretical Aspects of Reasoning about Knowledge, ed. M.Y. Vardi (Morgan Kaufmann, 1988) pp. 277?293.
4. A.M. Frisch and P. Haddawy, Probability as a modal operator, in:Proc. 4th AAAI Uncertainty in Artificial Intelligence Workshop (1988) pp. 109?118. A revised version by P. Haddawy and A.M. Frisch, entitled ?Modal logics of higher-order probability?, will appear inUncertainty in Artificial Intelligence 4, eds. R. Shachter, T.S. Levitt, J. Lemmer and L.N. Kanal (Elsevier Science, 1990).
5. R. Fagin, J.Y. Halpern and N. Megiddo, A logic for reasoning about probabilities, Info. Comput. 87 (1990) 78?128.