You may find yourself writing code that refers to
   variables and functions in base classes. This is
   particularly true if your derived class is a refinement
   or specialisation of code in your base class. 
  
   Instead of using the literal name of the base class in your
   code, you should be using the special name
   parent, which refers to the name of your
   base class as given in the extends
   declaration of your class. By doing this, you avoid using the
   name of your base class in more than one place. Should
   your inheritance tree change during implementation, the
   change is easily made by simply changing the 
   extends declaration of your class.