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One of the Zermelo-Fraenkel axioms of set theory, which states that two sets are equal precisely if they share all their elements: \(\forall x:\forall y: x=y\Leftrightarrow(\forall a:a\in x\Leftrightarrow a\in y)\). This axiom means that, for example, \(\{x,y\}=\{y,x\}=\{x,x,y\}\) i.e. ordering and repeats do not affect the definition of a set.