Visitor

Visitor: add operations over a fixed structure via double dispatch

Problem: you have a fixed set of node types and keep needing new operations over them — and you don't want to edit every node class each time.

PawWalk scenario: booking line items. A Total visitor sums them; a Receipt visitor formats them. Add a new visitor without touching the items.

Be honest: Visitor is clunky in dynamic languages. In real Ruby you'd often replace it with a case on type or pattern matching. It shines only when the node set is truly fixed and operations grow often.