private static IEnumerable<string> CreateRoman(string hiragana, int startIndex)
{
    var sb = new StringBuilder();
    for (var i = startIndex; i < hiragana.Length; i++)
    {
        switch (hiragana[i])
        {
            case 'あ':
                sb.Append('a');
                break;
            case 'い':
                sb.Append('i');
                break;
            case 'う':
                if (hiragana[i + 1] == '゛')
                {
                    var s = sb.ToString();
                    foreach (var x in CreateRoman(hiragana, i + 2))
                        yield return string.Concat(s, "vu", x);

                    char y;
                    switch (hiragana[i + 2])
                    {
                        case 'ぁ':
                            y = 'a';
                            break;
                        case 'ぃ':
                            y = 'i';
                            break;
                        case 'ぇ':
                            y = 'e';
                            break;
                        case 'ぉ':
                            y = 'o';
                            break;
                        default:
                            yield break;
                    }
                    sb.Append('v');
                    sb.Append(y);
                    s = sb.ToString();
                    foreach (var x in CreateRoman(hiragana, i + 3))
                        yield return string.Concat(s, x);
                }
                sb.Append('u');
                break;
            case 'え':
                sb.Append('e');
                break;
            case 'お':
                sb.Append('o');
                break;
        }
    }
    yield return sb.ToString();
}