public static function flash(string $key, $value = null)
  {
    /**
     * Check if session already has variables.
     *
     * If variables have already been set, merge the new
     * variables with the old ones.
     */
    return $_SESSION['application']['with'] = array_merge(
                                                isset($_SESSION['application']['with']) ? $_SESSION['application']['with'] : [],
                                                [$key => $value]
                                              );
  }