Visitor combination and traversal control

The Visitor design pattern allows the encapsulation of polymorphic behavior outside the class hierarchy on which it operates. A common application of Visitor is the encapsulation of tree traversals. A clean separation can be made between the generic parts of the combinator set and the parts that are specific to a particular class hierarchy. The generic parts form a reusable framework. The generic parts form a reusable framework. The specific parts can be generated from a (tree) grammar. Due to this separation, programming with visitor combinators becomes a form of generic programming with significant reuse of (visitor) code.