/**
  * sets current calendar user
  * 
  * @param Calendar_Model_Attender $_calUser
  * @return Calendar_Model_Attender oldUser
  */
 public function setCalendarUser(Calendar_Model_Attender $_calUser)
 {
     if (!in_array($_calUser->user_type, array(Calendar_Model_Attender::USERTYPE_USER, Calendar_Model_Attender::USERTYPE_GROUPMEMBER))) {
         throw new Tinebase_Exception_UnexpectedValue('Calendar user must be a contact');
     }
     $oldUser = $this->_calendarUser;
     $this->_calendarUser = $_calUser;
     $this->_eventController->setCalendarUser($_calUser);
     return $oldUser;
 }