Completed ex12
Completed ex12, study drill 1
Completed ex12, study drill 2
Completed ex13
Completed ex13, study drill 2
Completed ex13, study drill 3
Completed ex14
Completed ex14, study drill 2
Completed ex14, study drill 3
Completed ex15
Completed ex15, study drill 1
Revert "Completed ex15, study drill 1"

This reverts commit 417bb322397a4277d8636ee13f3f68498fb8a2c4.