1. Anderson, F. W. andFuller, K. R.,Rings and Categories of Modules, Springer Verlag, New York, Heidelberg and Berlin, 1973.
2. Birkhoff, G.,Lattice Theory, third ed., Amer. Math. Soc. Colloquium Publications 25, Amer. Math. Soc., 1967.
3. Burris, S. andSankappanavar, H. P.,A Course in Universal Algebra, Springer, 1981.
4. Cz�dli, G. andHutchinson, G.,An irregular Horn sentence in submodule lattices, Acta Sci. Math. (Szeged)51 (1987), 35?38.
5. Freyd, P. J.,Abelian Categories, An Introduction to the Theory of Functors, Harper and Row, 1964.