[1] J. Niederle, L. Polák:
The category of non-indexed algebras and weak homomorphisms. Colloquium Math. XLI (1979), 167-179.
MR 0591920
[2] L. Polák:
Multiplicity types of algebras-chain algebras. Finite Algebгa and Multiple-valued Logic, Szeged, Hungaгy, Colloquia Math. Soc. János Bolyai 28 (1979), 573-585.
MR 0648634