1. G. M. Bergman,Hyperidentities of groups and semigroups, Aeq. Math.23 (1981) 50?56.
2. P. M. Cohn,Universal Algebra, rev. edition, Reidel, Dordrecht 1981.
3. T. Evans,Some remarks on the general theory of clones, Colloq. Math. Soc. Janos Bolyai28 (1979) 203?244.
4. G. Gr�tzer,Universal Algebra, 2nd edition, Springer, New York (1979).
5. M. Hall Jr.,The Theory of Groups, Macmillan, New York 1959.