Exercises:
* [X] 2.1
* [X] 2.2
* [X] 2.3
* [X] 2.4
* [X] 2.5
* [X] 2.6
* [X] 2.7
* [X] 2.8
* [X] 2.9
* [X] 2.10
* [X] 2.11