Finish exercise 1
Finish exercise 2
Finish exercise 4
Finish exercise 6
Revert "Finish exercise 6"

This reverts commit f42fcbd415dc362066826557165e197f1806a390.
Revert "Revert "Finish exercise 6""

This reverts commit adbe27f742c8b8de9026b2ad03ee7844ae241713.
Add failing test for last exercise
Finish chapter 9 exercises