Abstraction and Functionality

I have added functionality to the classes and methods which have not already been implemented. You will notice that nothing at all has changed in the Transaction and Recompute classes. This is because they are already fully implemented! By using objects in this way, we can use the doTransaction method of each non-abstract child of BankAccount as a universal command-sending method (with very simple messages).

Obviously, feel free to change the implementations as you see fit: this is more your assignment than it is mine.

~polygon