Update my answer to Exercise 2.17.

Modified are the following 4 files:

	Exercise_2.17.pdf
	Exercise_2.17.tm
	Exercise_2.17.scm
	Test_for_Exercise_2.17.scm

Thank you!
							Lawrence X. Amlord
							  Nov 9th, 2013
							  Xi'an, China
Update my answer to Exercise 2.18.

Add an alternative implementation for last-element.

Modified are the following 4 files:

	Exercise_2.18.pdf
	Exercise_2.18.scm
	Exercise_2.18.tm
	Test_for_Exercise_2.18.scm

Thank you!
							Lawrence X. Amlord
							  Nov 9th, 2013
							  Xi'an, China