ex 1.13
Merge pull request #11 from OrazioWE7/master

ex 1.13
Exercise 1.19 answers
Merge pull request #12 from james-we7/master

Exercise 1.19 answers
exercise 21
Merge github.com:grahaminn/SICP
answer to ex 1-22
answer to ex 1-22
ex1.20
Merge branch 'master' of github.com:GusTheSadGeek/SICP
Merge pull request #13 from GusTheSadGeek/master

EX1.20