code: @Override
  	public void clearProperty(String key) {
          if (config instanceof ConcurrentCompositeConfiguration) {
              ((ConcurrentCompositeConfiguration) config).clearOverrideProperty(key);
          } else {
  		    config.clearProperty(key);
          }
  	}
docs: Calls <code>config.clearProperty()</code>. If the underlying configuration
  is {@link ConcurrentCompositeConfiguration}, it calls {@link ConcurrentCompositeConfiguration#clearOverrideProperty(String)}
  instead.
  <p><b>Warning: </b>{@link ConcurrentCompositeConfiguration#clearOverrideProperty(String)} does not clear the
  property with the whole {@link ConcurrentCompositeConfiguration}, if any other child configurations in it has the same property.