[1] Birkhoff G.: 
Lattice Theory. 3rd ed., Amer. Math. Soc. Colloq. Publ. 15, Providence, RI, 1973. 
MR 0227053[2] Chajda I.: 
Algebraic Theory of Tolerance Relations. Monograph Series of Palacký University, Olomouc, 1991, 117 p. 
Zbl 0747.08001[3] Zelinka B.: 
Representation of undirected graphs by anticommutative conservative groupoids. Math. Bohem. 119 (1994), 231-237. 
MR 1305526 | 
Zbl 0828.20074[4] Zelinka B.: Tolerance on conservative groupoids. Proceedings of Mathematics in Liberec 1993, VŠST Liberec 1993.
[5] Zelinka B.: 
Tolerances and congruences on conservative groupoids. Proceedings of the Summer School of General Algebra and Ordered Sets 1994, Publ. Palacký Univ., Olomouc 1995, 168-171. 
MR 1342554 | 
Zbl 0828.20073