Implementation of a list data structure using a visitor pattern.
Inspired by the book A Little Java, A Few Patterns by Matthias Felleisen and Daniel P. Friedman.
See last chapter of book on the Pieman.
com.boes.patterns: Main test class
com.boes.patterns.type: List, Node, and Visitor type interfaces
com.boes.patterns.structure: List, Node, and Empty data structures
com.boes.patterns.visitor: Occurs, Remove, Substitute, and String visitor behaviors