The book: fixed assertions in "validation > getters" example.

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all (or 2.2+)
| Fixed tickets | 

Please see "The Book" > Chapter: "Validation" > Section: "Getters".
There is a switcheroo in the code example - the assertion should the be that the method's return value must be "False", not "True".