[1] BELL J. L., SLOMSON A. B.: Models and Ultraproducts. North-Holland Publishing Company, Amsterdam, London 1969. MR 0269486 | Zbl 0179.31402

[2] GRÄTZER G.: General Lattice Theory. Akademie-Verlag, Berlin 1978.

[3] LIHOVÁ J., REPASKÝ K.: Congruence relations on and varieties of directed multilattices. Math. Slovaca 38, 1988, 105-122. Zbl 0642.06003