private function _getHelper($helper)
  {
    $instance = null;
    foreach ($this->_prefixes as $prefix) {
      $className = $prefix . "\\" . $helper;
      if (Loader::loadClass($className)) {
        if (is_subclass_of($className, __NAMESPACE__ . "\\Helper_Interface")) {
          $instance = new $className();
          /* @var $instance Helper_Interface */
          $instance->setSubject($this->_subject);
        }
        break;
      }
    }
    return $instance;
  }