Name:
  Visitor

Intent:
  - extend the functionality of a family of classes
    without modifying the classes themselves
  - functionality is often virtual
  - the family of classes is often a composite

Structure:
  - there is a method call "accept" in the base class of the family
  - there is an interface "Visitor" with an overloaded method "visit"
    for each concrete subclass of the family
  - method "accept" is overriden in each concrete subclass of the family
  - sometimes "visit" calls "accept"

Notes:
  - modifying the family becomes much more difficult
  - it looks like virtual method are pulled out of thei classes
  - must understand differences betwen overload and override
  - also called double dispatch