Open Closed Principle

classes / modules / functions (ENTITIES  ) should be open for extension,
but closed for modification. When a single change to a program results in
a cascade of changes to dependent modules, that program exhibits the
undesirable attributes that we have come to associate with “bad” design.
The program becomes fragile, rigid, unpredictable and unreusable.
The openclosed principle  says that you should design modules that never change.
When requirements change, you extend the behavior of such modules
by adding new code, not by changing old code that already works.