open-closed principle
The Open-Closed Principle (OCP) states that software entities
(classes, modules, methods, etc.) should be open for extension,
but closed for modification.
The parent/base classes to be used are OPEN only for extension (subclassing)
but these parent/base classes can not be modified (CLOSED for modification)
If a base class is modified other than just to improve performance, the latter
versions of code may actually try to use 2 different base classes with same name.