public static string Begin(this FontStyle fontStyle) {
switch (fontStyle) {
case FontStyle.Normal:
return "";
case FontStyle.Bold:
return "<b>";
case FontStyle.Italic:
return "<i>";
case FontStyle.BoldAndItalic:
return "<b><i>";
default:
throw new InvalidEnumArgumentException(nameof(fontStyle), (int) fontStyle, fontStyle.GetType());
}
}