Add my answer to Exercise 2.26.

Included are the following 3 files:

	Exercise_2.26.pdf
	Exercise_2.26.scm
	Exercise_2.26.tm

Thank you!
							Lawrence R. Amlord
							   May 30th, 2013
							   Xi'an, China
Update my answer to Exercise 2.18.
Fix typos in Exercise_2.18.scm.
Add my answer to Exercise 2.27.

Included are the following 4 files:

	Exercise_2.27.pdf
	Exercise_2.27.scm
	Exercise_2.27.tm
	Test_for_Exercise_2.27.scm

Thank you!
							Lawrence R. Amlord
							   May 30th, 2013
							   Xi'an, China