public function principalInfo($principal_id)
    {
        $response = $this->call('principal-info', array(
            'principal-id' => $principal_id,
        ));

        return array(
            'contact' => current($response->xpath('/results/contact')),
            'manager' => current($response->xpath('/results/manager')),
            'preferences' => current($response->xpath('/results/preferences')),
            'principal' => current($response->xpath('/results/principal')),
        );
    }