/// <summary>
 /// Retrieves the value of the attribute having the given name, or <c>null</c> if there is no such attribute.
 /// </summary>
 /// <param name="element"></param>
 /// <param name="name"></param>
 /// <returns></returns>
 public static string AttributeValue(this IXPathNavigable element, XName name)
 {
     return(element.AttributeValueOpt(name).SingleOrDefault());
 }