public static MappingDataNode Add(this MappingDataNode mapping, string key, DataNode node)
 {
     mapping.Add(new ValueDataNode(key), node);
     return(mapping);
 }
 public static MappingDataNode Add(this MappingDataNode mapping, string key, List <string> sequence)
 {
     mapping.Add(new ValueDataNode(key), new SequenceDataNode(sequence));
     return(mapping);
 }