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B.21. Writing a Visitor

Writing your own visitor can be quite easy if you use the provided facilities. If the normal method of iterating the language tree is sufficient, then follow these steps:
Create a subclass of AbstractLanguageVisitor. Override any visit methods needed for your processing. For instance, if you wanted to count the number of elements in the tree, you need only override the visit(ColumnReference) method. Collect any state in local variables and provide accessor methods for that state.
Decide whether to use pre-order or post-order iteration. Note that visitation order is based upon syntax ordering of SQL clauses - not processing order.
Write code to execute your visitor using the utility methods on DelegatingHierarchyVisitor: 
// Get object tree 
LanguageObject objectTree = ...

// Create your visitor initialize as necessary
MyVisitor visitor = new MyVisitor();

// Call the visitor using pre-order visitation
DelegatingHierarchyVisitor.preOrderVisit(visitor, objectTree);

// Retrieve state collected while visiting
int count = visitor.getCount();