private static string GetBulbLetter(BulbBase bulb)
        {
            if (!bulb.IsOn)
            {
                return("O");
            }

            switch (bulb.Color)
            {
            case BulbColor.Red:
                return("R");

            case BulbColor.Yellow:
                return("Y");

            default:
                throw new InvalidOperationException();
            }
        }
 private static void PrintBulb(StringBuilder output, BulbBase bulb)
 {
     output.Append(GetBulbLetter(bulb));
 }