add solution to Problem 2 in Problem Set 4.

Augmented are the following 2 files:

	src/sat/formula/Formula.java
	src/sat/formula/FormulaTest.java

The spec of the method CONTAINS in the ImList interface has
been revised:

	src/immutable/ImList.java

Thank you!
							Lawrence X. Amlord
							  Oct. 30th, 2014
							   Xi'an, China