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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