/** Let's make a tree, and print it out.
 *  There are 2 constructors we can call -- we can create a tree with a
 *  single node, or with 2 existing trees and a new node , we can make a 
 *  bigger tree.
 */
import java.util.*;

public class Driver
{
  public static void main(String [] args)
  {
    Tree one = new Tree(new Item("1"));
    Tree two = new Tree(new Item("2"));
    Tree three = new Tree(new Item("3"));
    Tree four = new Tree(new Item("4"));
    Tree difference = new Tree(new Item("-"), one, two);
    Tree product = new Tree(new Item("*"), three, four);
    Tree sum = new Tree(new Item("+"), difference, product);

    // default printout (preorder shown vertically)
    System.out.println(sum);
  }
}