Ch15 - Stub out Sum plus

Chapter 15: Mixed Currencies, page 75
Ch16 - Add test for Sum plus

Chapter 16: Abstraction, Finally, page 77
Ch16 - Complete Sum plus implementation

Chapter 16: Abstraction, Finally, page 77
Ch16 - Add test for Sum times

Chapter 16: Abstraction, Finally, page 78
Ch16 - Add Sum times implementation

Chapter 16: Abstraction, Finally, page 78