Union (set theory) - Wikipedia
https://en.wikipedia.org/wiki/Union_(set_theory)
Similarly, union is commutative, so the sets can be written in any order. The empty set is an identity element for the operation of union. That is, A ∪ ∅ = A, for any set A. This follows from analogous facts about logical disjunction. Since sets with unions and intersections form a Boolean algebra, intersection distributes over union
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