abstract class ExpTree {
    private Object value;
    private ExpTree[] children;
 
// Constructors

    public ExpTree(Object v) {                // Only the value
         value = v;
       }

    public ExpTree(ExpTree a, ExpTree b) {   // Two subtrees
         children = new ExpTree[2];
         children[0]=a;
         children[1]=b;
       }
         
    public ExpTree(ExpTree[] c) {            // Array of subtrees
              children=c;
     }

// Getters

    public Object getValue() { return value; }
    public ExpTree[] getChildren() { return children; }

    public abstract Object evaluate(); 
    public abstract String display();
   }