In a rooted tree, the "parent" of a vertex "v" is the vertex connected to "v" on the path to the root; every vertex has a unique parent except the root which has no parent. 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". A "sibling" to a vertex "v" is any other vertex on the tree which has the same parent as "v". A "leaf" is a vertex with no children. An "internal vertex" is a vertex that is not a leaf.