public function onUpdate( \Niirrty\Observer\IObservable $observable, $extras = null )
   {
      echo '- Observable change property ';
      if ( \is_array( $extras ) && isset( $extras[ 'property' ] ) )
      {
         echo '"' . $extras[ 'property' ], '" ';
      }
      echo "to a new value\n";
   }