1. S. L. Bloom, Some theorems on structural consequence operations, Studia Logica 34 (1976), pp. 1?9.
2. C. C. Chang and H. J. Keisler, Model Theory, North-Holland and American Elsevier, Amsterdam-London-New York, 1973.
3. G. Gr�tzer, Universal Algebra, Van Nostrand, Princeton, New Jersey, 1968.
4. G. Gr�tzer and H. Lakser, Some new relations on operators in general, and for pseudocomplemented distributive lattices in particular, Notices of the American Mathematical Society, 17 (1970), p. 642.
5. J. ?o?, O matrycach logicznych (On logical matrices, in Polish), Travaux de la Soci�t� des Sciences et des Lettres de Wroc?aw, Seria B, Nr 19, Wroc?aw 1949.