docs: private static void _validateConfigurationHierarchy (@Nonnull final Configuration aConfig)
    {
      final ICommonsSet <String> aHierarchySet = new CommonsHashSet <> ();
      for (final Configuration aSubConfig : aConfig.getSubConfigurations ())
      {
        final String sHierarchy = aSubConfig.getHierarchy ();
        if (!aHierarchySet.add (sHierarchy))
          throw new IllegalArgumentException ("Configuration for " +
                                              aConfig.getHierarchy () +
                                              " contains multiple SubConfigurations with the same hierarchy id '" +
                                              sHierarchy +
                                              "'. ");
      }
  
      for (final Configuration aSubConfig : aConfig.getSubConfigurations ())
        _validateConfigurationHierarchy (aSubConfig);
    }