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2. ?, Modal Logic as Second-Order Logic, report 77-04, 1977, Department of Mathematics, University of Amsterdam; expanded version to appear in Studia Logica Selected Topics Library 1980.
3. ?, Canonical modal logics and ultrafilter extensions, The Journal of Symbolic Logic 44 (1979), pp. 1?8.
4. ?, Two simple incomplete modal logics, Theoria 44 (1978), pp. 25?37.
5. C. C. Chang and H. J. Keisler, Model Theory, North-Holland, Amsterdam, 1973.