 A "child" of a vertex "v" is a vertex of which "v" is the parent.An "ascendant" of a vertex "v" is any vertex which is either the parent of "v" or is (recursively) the ascendant of the parent of "v".A "descendant" of a vertex "v" is any vertex which is either the child of "v" or is (recursively) the descendant of any of the children of "v".