Name:
  Decorator

Intent:
  - avoid a combinatorial explosion of subclasses
  - extend (and retract) the functionality of objects at run-time

Structure:
  - there a decorator interface
  - subject has a reference to a decorator
  - the decorator interface has a reference to a decorator
  - subject delegates activities to its decorator
  - sometimes there is a null object decorator

Notes:
  - the chain can be forward (subject first) or backward (subject last)
  - the chain of decorators can be build dynamically