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Design pattern: Visitor

Description: "Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates."

This pattern can be used with both classes and prototypes.

This design pattern can be trivially implemented by defining a suitable meta-predicate that walks the structure and applies a user defined closure to each element. When the object defining the structure doesn't provide such a meta-predicate, a complementing category can define and add it to the object. Both cases are illustrated in the sample code.