A set is a collection which is unordered and unindexed, set items are unchangeable. They are useful to remove duplicate values and to perform operations like unions and intersections. The official documentation.

Different kind of sets

The station you are trying to open is now closed, but there are several ways which will allow you to open it.

Authorize

In order to get the maximum of the game, you should login. During this process you can use your social accounts such as FaceBook, Twitter, G+, Github, StackOverflow, etc.