At the beginning of the book we said that computer programs are about processing some input and generating some output from it, possibly also altering the state of some data-holding instance like a file or a database. Although this is certainly true, it does not tell the whole story. In real-world scenarios, computer programs exhibit another characteristic: They are supposed to be of some practical use, and as a result of that, model real-world events and things.
Say, for example, you write a simple program for registering paper invoices and summing up ...
