Added the code for exercise 11.1
Added the code for exercise 11.2
Added the code for exercise 12.1
Added the code for exercise 12.2
Added the code for exercise 12.3
Added the code for exercise 13.1
Added the code for exercise 13.2
Added the code for exercise 13.3
Added the code for exercise 13.4
Added the code for exercise 13.5
Added the code for exercise 14.1