protected function parseAction($fields, $default) {
    if (empty($fields[self::PREFIX]['action'])) {
      return $default;
    }
    else {
      if (is_array($fields[self::PREFIX]['action'])) {
        foreach ($fields[self::PREFIX]['action'] as $name => $label) {
          return $name;
        }
      }
      elseif (is_string($fields[self::PREFIX]['action'])) {
        return $fields[self::PREFIX]['action'];
      }
    }
    return NULL;
  }