[2] J. Duda: 
On two schemes applied to Mal’cev type theorems. Ann. Univ. Sci. Budapest, Sect. Math. 26 (1983), 39–45. 
MR 0719774 | 
Zbl 0518.08002[3] J. Duda: 
Varieties having directly decomposable congruence classes. Čas. Pěst. Matem. 111 (1986), 394–403. 
MR 0871715 | 
Zbl 0606.08001