Update my answer to Exercise 2.73.

Modified are the following 3 files:

	Exercise_2.73.pdf
	Exercise_2.73.scm
	Exercise_2.73.tm

Newly added is the following file:

	Test_of_Exercise_2.73.scm

Thank you!
							Lawrence R. Amlord
							   Aug 6th, 2013
							   Xi'an, China