Update Code in section 2.5.3-Example: Symbolic Algebra

Modified is the following file:

	Example-Symbolic_Algebra.scm

Newly added is the following file:

	Test_of_Example-Symbolic_Algebra.scm

Thank you!
						Lawrence R. Amlord
						   Aug 16th, 2013
						   Fuzhou, China
Update my answer to Exercise 2.80.

Modified are the following 2 files:

	Exercise_2.80.pdf
	Exercise_2.80.tm

Thank you!
							Lawrence R. Amlord
							   Aug 16th, 2013
							   Fuzhou, China
Update my answer to Exercise 2.87.

Modified are the following 2 files:

	Exercise_2.87.pdf
	Exercise_2.87.tm

Besides, I've also added 2 files for explanation:

	Exercise_2.87.scm
	Test_of_Exercise_2.87.scm

Thank you!
							Lawrence R. Amlord
							   Aug 16th, 2013
							   Fuzhou, China