public function addConnection($dsn, $username= '', $password= '', array $options= array(), $driverOptions= null) {
        $added = false;
        $connection= new xPDOConnection($this, $dsn, $username, $password, $options, $driverOptions);
        if ($connection instanceof xPDOConnection) {
            $this->_connections[]= $connection;
            $added= true;
        }
        return $added;
    }