Chapter 4 Exercises -axp

The point of this is that if we don't explicitly specify a return value, programmer A might write code that depends on `greet` returning `$name`. Programmer B might then extend `greet` with further code (logging or whatever), and accidentally break programmer A's code.

This could also be mitigated by explicitly documenting the return value for `greet`, but it's generally better for code to self-document.