/// <summary>
 /// Add some layerages into children.
 /// </summary>
 /// <param name="layerageCollection"> The layerage-collection. </param>
 /// <param name="currentLayerage"> The current layerages. </param>
 /// <param name="layerages"> The source layerages. </param>
 public static void AddRange(LayerageCollection layerageCollection, Layerage currentLayerage, IEnumerable <Layerage> layerages) => LayerageCollection._add(layerageCollection, currentLayerage, null, layerages);
 /// <summary>
 /// Add a current-layerage into source-layerage's children.
 /// </summary>
 /// <param name="layerageCollection"> The layerage-collection. </param>
 /// <param name="currentLayerage"> The current layerage. </param>
 /// <param name="layerage"> The source layerage. </param>
 public static void Add(LayerageCollection layerageCollection, Layerage currentLayerage, Layerage layerage) => LayerageCollection._add(layerageCollection, currentLayerage, layerage, null);