Editing Visitor pattern (section)

=== Problems, the Visitor design pattern can solve ===

* It should be possible to define a new operation for (some) classes of an object structure without changing the classes.

When new operations are needed frequently and the object structure consists of many unrelated classes,
it's inflexible to add new subclasses each time a new operation is required
because "[..] distributing all these operations across the various node classes leads to a system that's hard to understand, maintain, and change."<ref name="GoF" />