/// <summary>
/// This method returns the list of prokaryotes and its respective class levels.
/// Throws an exception if any error accessing the database occurs.
/// </summary>
public static XmlDocument listProkaryotes()
{
XmlDocument doc = new XmlDocument();
doc.AppendChild(dictionaryToXML(GDDL.listProkaryotes(), doc));
return(doc);
}