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One of the Zermelo-Fraenkel axioms of set theory, which states that every non-empty set \(x\) contains an element which is disjoint from \(x\): \(\forall x:x\neq\emptyset\Rightarrow (\exists a\in x:a\cap x=\emptyset)\). Important consequences are that no set may contain itself, nor can there exist 'cycles' in the epsilon relation (i.e. a sequence of sets \(x_1,...,x_n\) satisfying \(x_1\in x_2\in \cdots\in x_n\in x_1\), nor can there exist a chain of 'infinite descent' (an infinite sequence \(x_1,x_2,...\) such that \(x_1\ni x_2\ni\cdots\)).