1. Blok, W. J. andPigozzi, D.,Algebraizable logics, Memoirs of the AMS Vol.77 (1989),No. 396.
2. Blok, W. J. andPigozzi, D.,On the structure of varieties with equationally definable principal congruences III, to appear in Algebra Universalis.
3. Burris, S. andSankappanavar, H. P.,A Course in Universal Algebra, Graduate Texts in Mathematics, Springer-Verlag, New York 1981.
4. Chajda, I.,Tolerances in congruence permutable algebras, Czechoslovak Math. J.38 (1988), 218?225.
5. Cornish, W. H.,Varieties generated by finite BCK-algebras, Bull. Austral. Math. Soc.22 (1980), 411?430.