/** class Sum, our internal representation of a sum-expression
 * in the L0 language.
 * See http://www.radford.edu/itec380/2008fall/Hw06/hw06.html
 * 
 * @author Ian Barland
 * @version 2008.Nov.30
 */
public class Sum extends Expr {
  static final String TOKEN = "+";
  
  Expr e1, e2;
  Sum( Expr _e1, Expr _e2 ) {
    this.e1 = _e1;
    this.e2 = _e2;
    }
  
  public String toString() {
    return "(" + e1.toString() + " " + TOKEN + e2.toString() + ")";
    }
    
  public Value eval() {
    return new Num( ((Num)(e1.eval())).doubleValue()
                  + ((Num)(e2.eval())).doubleValue() );
    }
  }