public function clean($value)
  {
    if ($value==='' || $value===null || $value===false)
    {
      return null;
    }

    $tmp = PruneWhitespaceCleaner::get()->clean($value);
    if ($tmp==='' || $tmp===null)
    {
      return null;
    }

    return mb_substr($tmp, 0, $this->maxLength);
  }