public function setDocumentDistributorCallback($callback)
    {
        if ($callback !== null  &&  !is_callable($callback)) {
            include_once 'Zend/Search/Lucene/Exception.php';
            throw new Zend_Search_Lucene_Exception('$callback parameter must be a valid callback.');
        }

        $this->_documentDistributorCallBack = $callback;
    }