Dear head from Lascaux and a "dear frieze" from Altamira.

In a set A the relation of equivalence ~ makes its partition into equivalence classes. Every class of equivalence consists of the elements from A connected by the relation ~. The properties of equivalence classes are: (a) each class is not empty; (b) every two classes are disjoint; (c) the union of all classess is the complete set A. Any element from an equivalence class can be equaly chosen as its representative.

By connecting every element of the set A with its equivalents, we obtain the graph-interpretation of the partition, where the elements of different classes of equivalence belong to mutually disconnected parts (i.e. there is no path from one to another). Elements having no equivalents are singular: each of them makes its own equivalence class.

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