Update the code in section 4.1.2.

Modified is the following file:

	4.1.2_Representing_Expressions.scm

Thank you!
							Lawrence X. Amlord
							  Jan 16th, 2013
							  Fuzhou, China
Update my answer to Exercise 4.2.

Modified are the following 2 files:

	Exercise_4.2.pdf
	Exercise_4.2.tm

Thank you!
							Lawrence X. Amlord
							  Jan 16th, 2014
							  Fuzhou, China