More exercises
Begin 5-stars exercise

Some attempts here are commented out and will be removed in next
commit.
Remove most non-working code

I preserved the non-working definition of division, since it is called
for in next exercise.
WIP on exercise, many attempts now to remove
Cleanup current status
Finish proofs successfully - still to cleanup
Cleanup proofs and complete Chapter 1!

I only skipped writing informal proofs in this chapter.
Complete chapters 2 and 3 (except informal proofs).