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