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# Chapter 9. Sets

A set is an unordered collection of distinct members. The basic operations on a set are testing for membership, adding members, and removing members. Other operations include set union, intersection, difference, and symmetric difference. Given two sets `s` and `t`, the union `s` + `t` is a set that contains everything in `s` and everything in `t`; the intersection `s` * `t` is the set whose members appear in both `s` and `t`, the difference `s` - `t` is the set whose members appear in `s` but not in `t`, and the symmetric difference, often written as `s` / `t`, is the set whose members appear in only one of `s` or `t`.

Sets are usually described in terms of a universe — the set of all possible members. For example, sets of characters are usually associated with the ...

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