1. Hedrl�n, Z., and A. Pultr: On full embeddings of categories of algebra. Illinois J. Math.10, 392?406 (1966).
2. Pultr, A.: On selecting of morphisms among all mappings between underlying sets of objects in concrete categories and realisation of these. Comment. Math. Univ. Carolinae8, 53?83 (1967).
3. Vop?nka, P., A. Pultr, and Z. Hedrl�n: A rigid relation exists on any set. Comment. Math. Univ. Carolinae6, 149?155 (1965).