Intuitively: unordered container where all the values are unique, just like C++

`std::set`

.More precisely for set theory formalization of Mathematics:

- everything is a set, including the elements of sets
- string manipulation wise:
`{}`

is an empty set. The natural number`0`

is defined as`{}`

as well.`{{}}`

is a set that contains an empty set`{{}, {{}}}`

is a set that contains two sets:`{}`

and`{{}}`

`{{}, {}}`

is not well formed, because it contains`{}`

twice

- Formalization of Mathematics | 457, 866, 25
- Mathematics | 17, 13k, 336
- Ciro Santilli's Homepage | 262, 182k, 3k

- Function | 27, 79, 4