This example shows how computed observables can be chained together.
Each cart line has a ko.computed property for its own subtotal, and these in turn are combined in a further ko.computed
property for the grand total. When you change the data, your changes ripple out through this chain of computed properties,
and all associated UI is updated.
This example also demonstrates a simple way to create cascading dropdowns.
In the sample above there is no meaning to the Category, it has meaning with the Price and (5%) Tax.
In the sample below changes to Category reset the missing Product selection.