1. Bloom S. L., Some theorems on structural consequence operations, Studia Logica, vol. 34, pp. 1–10, 1975.
2. Brown D. J., Suszko R., Abstract logics, Dissertationes Mathematicae, vol. 102, 1973.
3. Chang C.C., Keisler H. J., Continuous model theory, Princeton 1966.
4. Chang C.C., Keisler H. J., Model theory, North-Holland, Amsterdam, 1973.
5. Cohn P. M., Universal algebra, Harper and Row, New York and London, 1965.