You may want to use ExpressionVisitorXXXX class if you want to return boolean, void, or {@link Expression}.
It is the callee's responsibility to traverse child expression. Expression and its derived classes do not provide any traversal. See {@link ExpressionCloner} for example.
onRef method is called for all subclass of ReferenceExp. So you can safely use this interface to visit AGMs from RELAX grammar. @author Kohsuke KAWAGUCHI
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