The four code samples that fail to run are:
- Add attribute magic to objects. Fixed by introducing a Person
- Tracking value changes. Fixed by replacing `attr_accessor` with
`define_attribute_methods`, providing getter and setter methods
for `name` and providing the missing `Person#save` method. A
call to `Person#save` has to precede the `person.name = 'robert'`
assignment, if we want `previous_changes` to include 'bob'.
- Adding `errors` interface to objects. Fixed by introducing a
Person instance variable, assigning `nil` to its name and calling
- Custom validators. Fixed by defining `HasNameValidator` before
it is used by `ValidatorPerson`.
All the code samples can now be run smoothly.
Call Dirty#changes_applied in Person#save, instead of modifying instance vars.