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On full embeddings of categories of algebras. Illinois J. of Math. 10 (1966), 392-406. 
MR 0191858[3] P. VOPĚNKA A. PULTR Z. HEDRLÍN: 
A rigid relation exists on any set. Comment. Math. Univ. Carolinae, vol. 6 (1965), 149-155.  
MR 0183647