Add SICP excises 2.40
Add SICP excises 2.41
Add SICP excises 2.42
Add SICP excises 2.44
Add SICP excises 2.45
Add SICP excises 2.46
Add SICP excises 2.47
Add SICP excises 2.48
Add SICP excises 2.49
Add SICP excises 2.50