/* interface of Tree ADT */
public interface Tree<T extends Comparable<T>> {
    /* add an element to the tree */
    void add(T o);
    /* remove an element; returns the element if found else return null */
    T remove(T o);
    /* test if tree is empty */
    boolean isEmpty();
    /* returns instance of class that implements traversal interface */
    Traversal<T> traversal();
    /* number of elements */
    int size();
    /* height of the tree */
    int height();
    /* is the tree balanced */
    boolean balanced();
}