Optimize for sign cases. [2.11]
Remove comments. [2.11]
Merge branch 'ex2.11'
Additional functions from book. [2.12]
Add percent functions. [2.12]
Merge branch 'ex2.12'
Write last-pair. [2.17]
Merge branch 'ex2.17'
Write reverse. [2.18]
Merge branch 'ex2.18'
cc procedure from book. [2.19]
Implement remaining procedures. [2.19]
Merge branch 'ex2.19'