Follow the Alternation-Non-Enumeration rule:
   Avoid enumerating the subclasses in abstract class names. Reason:
   Each time you add a subclass, your abstract class name becomes
   outdated. Example: AddClassOrAttribute is better renamed to: AddItem.

   AddClassOrAttribute     : AddClass | AddAttribute .

   If we add AddMethod as a subclass, AddItem is still correct.

   AddItem     : AddClass | AddAttribute | AddMethod.