code: private static Configuration merge(Configuration one, Configuration two) {
      if (one == null) {
        if (two == null) {
          return new Configuration();
        }
        return new Configuration(two);
      }
  
      Configuration c = new Configuration(one);
  
      if (two == null) {
        return c;
      }
  
      for (Map.Entry<String, String> entry : two) {
        c.set(entry.getKey(), entry.getValue());
      }
  
      return c;
    }
docs: where Configuration.addResource(configuration) is not available