public static IOrderedVirtualForest <T> ThenChildrenByDescending <T, TKey>(
     this IOrderedVirtualForest <T> forest,
     Func <T, TKey> keySelector,
     IComparer <TKey> comparer)
 {
     return
         (forest
          .CreateOrderedVirtualForest(
              new OrderByComparer <T, TKey>(
                  keySelector,
                  comparer ?? Comparer <TKey> .Default,
                  true)));
 }
 public static IOrderedVirtualForest <T> ThenChildrenBy <T, TKey>(
     this IOrderedVirtualForest <T> forest,
     Func <T, TKey> keySelector,
     IComparer <TKey> comparer)
 {
     return
         (forest
          .CreateOrderedVirtualForest(
              new OrderByComparer <T, TKey>(
                  keySelector,
                  comparer,
                  false)));
 }