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Code for this example can be found on http://users.vnet.net/wwake.
The Test/Code Cycle in XP: Part 1, Model, William Wake, found on http://users.vnet.net/wwake.
 Copyright 2000, William C. Wake, used with permission. William.Wake@acm.org.
Extreme Programming Explained, Kent Beck.
Refactoring, Martin Fowler.
JUnit can be found on http://www.XProgramming.com.