public class Product implements Expression {
    protected Expression multiplier, multiplicand;
    public Product(Expression multiplier, Expression multiplicand) {
        this.multiplier = multiplier;
        this.multiplicand = multiplicand;
    }
    public Expression multiplier() { return multiplier; }
    public Expression multiplicand() { return multiplicand; }
    public Object visit(ExpressionVisitor v) {
        return v.visitProduct(this);
    }
    public String toString() {
        return "(" + multiplier.toString()
            + " * " + multiplicand.toString() + ")";
    }
}