1. Burris, S., Sankappanavar, H.P.: A Course in Universal Algebra, millennium edn. http://www.math.uwaterloo.ca/~snburris/htdocs/ualg (2000)
2. Denecke K., Koppitz J., Shtrakov Sl.: Multi-hypersubstitutions and colored solid varieties. Internat. J. Algebra Comput. 16, 797–815 (2006)
3. Denecke, K., Lau, D., P¨oschel, R., Schweigert, D.: Solidifyable clones. In: General Algebra, vol. 20, pp. 41–69. Heldermann, Berlin (1993)
4. Graczýnska, E.: On normal and regular identities and hyperidentities. In: Universal and Applied Algebra (Turawa, 1988), pp. 107–135. World Scientific (1989)
5. Shtrakov Sl., Denecke K.: Essential variables and separable sets in universal algebra. Mult.-Valued Log. 8, 165–181 (2002)