code: @Override
  	public void addProperty(String key, String value) {
          if (config instanceof ConcurrentCompositeConfiguration) {
              ((ConcurrentCompositeConfiguration) config).setOverrideProperty(key, value);
          } else {
  		    config.addProperty(key, value);
          }
  	}
docs: Calls <code>config.addrProperty()</code>. If the underlying configuration
  is {@link ConcurrentCompositeConfiguration}, it calls {@link ConcurrentCompositeConfiguration#setOverrideProperty(String, Object)}
  instead.