Axiom^{1} of Extension (аксиома на удължаването / разширението): a set is determined by what its elements are  not in the order in which they might be listed or the fact that some elements might be listed more than once.
We say that sets
Set equality is also a Relation between two sets.
Footnotes

axiom: a statement that is taken to be true to serve as a starting point for further reasoning and arguments ↩