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Closure
# definition of closure in set theory
$ \quad cl \quad : \ \mathcal{P}(X) \ \rightarrow \ \mathcal{P}(X) $ satisfies * $ X \subseteq cl(X) $ * $ X \subseteq Y \quad \Rightarrow \quad cl(X) \subseteq cl(Y) $ * $ cl(cl(X)) = cl(X) $