public function addColumn($key,$args = null)
    {
        if ($key instanceof Column) {
            $args = $key->getColumnArray();
            $key = $key->name;
        }
        if (array_key_exists($key,$this->columns))
            trigger_error(sprintf(self::TEXT_ColumnExists,$key),E_USER_ERROR);
        $column = new Column($key, $this);
        if ($args)
            foreach ($args as $arg => $val)
                $column->{$arg} = $val;
        // skip default pkey field
        if (count($this->pkeys) == 1 && in_array($key,$this->pkeys))
            return $column;
        return $this->columns[$key] =& $column;
    }