The store sells movies and lets the user rent/return them.
The store starts by adding movies to its collection.
A movie is described by its name, format, and rating.

               Test Cases
----------------------------------------
- adding a movie
    - `[identify test case 1]`
- selling a movie
    - `[identify test case 2]`
    - `[identify state exception test case]`
- renting a movie
    - `[identify test case 4]`
- returning a movie
    - `[identify test case 5]`