Our inventory system must keep track of stock on hand, and items that have been backordered but are not on hand. We can keep these in two lists. Just to illustrate a variety fo different uses, the on hand list will be a list of actual widget values, while the back order list will be a list of integers, namely the widget identification numbers.
The inventory system consists of two functions. The order function means that somebody has ordered a widget with the particular identification number, while the reciever means that we have received a widget in the widget wherehouse.