Chapter 3

Add equality test
Chapter 3

Succeed with equality test

- Add fake implementation
Chapter 3

Refactor fake implementation for equals()

Use triangulation to avoid gradual removal of duplication (replacing
true by 5 == 5, replacing by amount == 5, replacing by amount ==
other.amount).
Triangulation means write another test so that both tests together
demand a more general solution.
Chapter 4

Refactor multiplication test

Remove duplication by using new equals implementation

Reduce visibility of amount to private