Final Implementation. Not!

There is no "final" implementation. Let me reiterate:

There is no "final" implementation.

So how do we know when to stop? The truth is, we don’t. Sometimes we just get bored with the program, sometimes we start a new interesting project and we abandon the current one (promising ourselves that we’ll come back to it later). Having a delivery date planned ahead (even if we slip a little), and a manager who tells us to stop adding new features and ship—helps.

With this caveat in mind, let me go over the pseudo-final version of the calculator to show you how the stubs get expanded to create a working program.
Next.