public object OpMultiply(object lv, object rv)
{
if (lv == null || rv == null)
throw ErrorFactory.CreateTypeError( "Invalid operand type: 'null'");
if (lv is BikeNumber && rv is BikeNumber)
return new BikeNumber(((BikeNumber)lv).Value * ((BikeNumber)rv).Value);
throw ErrorFactory.CreateTypeError(string.Format("Invalid operand type: {0} * {1}", lv, rv));
}