public class Sum implements Expression {
    protected Expression addend, augend;
    public Sum(Expression addend, Expression augend) {
        this.addend = addend;
        this.augend = augend;
    }
    public Expression addend() { return addend; }
    public Expression augend() { return augend; }
    public Object visit(ExpressionVisitor v) {
        return v.visitSum(this);
    }
    public String toString() {
        return "(" + addend.toString() + " + " + augend.toString() + ")";
    }
}