A common pattern is a superclass with "abstract" methods that subclasses then implement. So the `sendCommand(cmd)` function is implemented in HPDriver and Canon

A common pattern is a superclass with "abstract" methods that subclasses then implement. So the `sendCommand(cmd)` function is implemented in HPDriver and CanonDriver and higher level methods in PrinterDriver can use that piece even though PrinterDriver itself has no implementation of that method.