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2. S. Burris and H. P. Sankappanavar, A Course in Universal Algebra, vol. 78 of Graduate Texts in Mathematics (Springer-Verlag, New York-Berlin, 1981).
3. A. Chagrov and M. Zakharyaschev, Modal Logic, vol. 35 of Oxford Logic Guides (The Clarendon Press, Oxford University Press, New York, 1997).
4. H. B. Curry, “On the definition of negation by a fixed proposition in inferential calculus,” J. Symbolic Logic 17, 98–104 (1952).
5. H. B. Curry, “The system LD,” J. Symbolic Logic 17, 35–42 (1952).