Given a tree rooted at
n, the visitor has the option of
overriding all traversal of the children of
n. If no
changes were made to
n and the visitor wishes to prevent
further traversal of the tree, then it should return
n. If
changes were made to the subtree, then the visitor should return a
copy of
n with appropriate changes. Finally, if the
visitor does not wish to override traversal of the subtree rooted at
n, then it should return
null.
This method is typically called by the method
override(parent, n). If a subclass overrides the
method override(parent, n) then this method
may not be called.