`AbstractSet` is a java class, and it brings flexible types with it.
Because of it, it was possible to write the following code:
```kt
fun usage() {
val mySet = SmartSet.create<String>() // should not contain nulls
mySet += null // compiles because of flexible types
}
```
Using `AbstractMutableSet`
as a base class does not change the implementation,
but explicitly adds a proper kotlin `MutableSet`
interface to the class, making it more type-safe
Roman Golyshev
648330da50