Here is the window we’ll be building at the right end.

The “Total” field at the bottom is really the computed price, quantity times unit price. (I learned the term “Extended Price” from the IBM trainers when I was doing punched-card accounting in 1956.)

One of the things you will see is that the arithmetic for doing this calculation is not in the flow diagram but in the flow objects themselves. This is a key idea for separating the application and domain layers, which I’ll be showing in a simplified way.