Update my answer to Exercise 4.6.

Modified is the following file:

	Exercise_4.6.scm

Thank you!
							Lawrence X. Amlord
							  Jan 18th, 2014
							  Fuzhou, China
Update my answer to Exercise 4.8.

This is a major revision, the answer has been rewritten heavily.

Modified is the following file:

	Exercise_4.8.scm

Thank you!
							Lawrence X. Amlord
							  Jan 18th, 2014
							  Fuzhou, China