public interface SortTree<E extends Comparable<E>> {
    public int size();			// number of elements
    public boolean contains(E elem);	// true, if elem in tree
    public void add(E elem);		// add elem to tree
    public void remove(E elem);	// tremove elem from tree
    public java.util.List<E> traverse();		// in-order traversal
    public void balance();		// balance the tree
}