// file Subtract.java

public class Subtract extends ExpTree {

   public Subtract(ExpTree a, ExpTree b) {
       super(a,b);
     }

   public Object evaluate() {
       return (Integer) getChildren()[0].evaluate()  -
              (Integer) getChildren()[1].evaluate();  
     }

   public String display() {
       return "(" + getChildren()[0].display() + " - " 
                  + getChildren()[1].display() + ")" ;
     }
}