public static IOrderedVirtualForest <T> ThenChildrenByDescending <T, TKey>(
     this IOrderedVirtualForest <T> forest,
     Func <T, TKey> keySelector)
 {
     return
         (forest
          .ThenChildrenBy(
              keySelector,
              null));
 }
 public static IOrderedVirtualForest <T> ThenChildrenBy <T, TKey>(
     this IOrderedVirtualForest <T> forest,
     Func <T, TKey> keySelector)
 {
     return
         (forest
          .ThenChildrenBy(
              keySelector,
              Comparer <TKey> .Default));
 }