Add my answer to Exercise 4.2.

Included are the following 2 files:

	Exercise_4.2.pdf
	Exercise_4.2.tm

Thank you!
							Lawrence X. Amlord
							  Dec 12th, 2013
							  Xi'an, China
Update my answer to Exercise 2.73.

Modified are the following 2 files:

	Exercise_2.73.pdf
	Exercise_2.73.tm

Thank you!
							Lawrence X. Amlord
							  Dec 14th, 2013
							  Xi'an, China