If you try to build a CopyVisitor of some kind of object hierarchy, one has to build some kind of Stack structure, to give the actual visitor a hint, where it should write its copied data.

I suggest to extend all visitor methods with a return value:

MyType visitor(MyType param);

The standard implementation should return the actual object inserted.

public interface StatementVisitor { Comment visit(Comment comment); Commit visit(Commit commit); Delete visit(Delete delete); Update visit(Update update);