public function prep(&$data, $flatten = self::PREP_HOP) {
    foreach($data as &$v) {
        $v = (is_array($v))
		  ? (($flatten & self::PREP_JOIN) 
		    ? join(',', prep($v)) 
			  : prep($v))
		    : $v = "'" . $v . "'";
    }  
    return $data;
  }