public function where_in($arg = null, $val = null, $operator = 'OR')
    {
        $this->where .=
            (strlen($this->where) ? " {$operator} " : '')
            .$arg
            .( !empty($val) && is_array($val) ? " IN (".self::escape( implode(',',$val) ).")" : '' );
        return $this;
    }