public function modifyRelationship($action, $user) {
        if (true === in_array($action, $this->_actions) && isset($user)) {
            return $this->_makeCall('users/' . $user . '/relationship', array('action' => $action), 'POST');
        }
        throw new InvalidParameterException('Error: modifyRelationship() - This method requires an action command and the target user id.');
    }