void IDL.updateTester(int id, Tester updatedTester, bool[,] updatedSchedule)
        {
            XElement testerElement = (from item in TesterRoot.Elements()
                                      where int.Parse(item.Element("ID").Value) == id
                                      select item).FirstOrDefault();

            if (testerElement == null)
            {
                throw new TestersIdNotFoundException(id);
            }
            testerElement.Remove();
            TesterRoot.Add(updatedTester.ToXmlTester());
            TesterRoot.Save(TesterPath);

            XElement scheduleXelement =
                ScheduleRoot.Elements()
                .Where(item => item.Element("ID").Value == id.ToString())
                .FirstOrDefault();

            if (scheduleXelement != null)
            {
                scheduleXelement.Remove();
            }

            XElement testersId = new XElement("ID", id.ToString());

            ScheduleRoot.Add(new XElement("TesterSchedule", testersId, updatedSchedule.ToXmlSchedule()));
            ScheduleRoot.Save(SchedulePath);
        }
        void IDL.addTester(Tester tester, bool[,] schedule)
        {
            int id = (from item in TesterRoot.Elements()
                      where (int.Parse(item.Element("ID").Value) == tester.ID)
                      select(int.Parse(item.Element("ID").Value))).FirstOrDefault();

            if (id != 0)
            {
                throw new TestersIdAlreadyExistsException(tester.ID, tester.FirstName + " " + tester.LastName);
            }

            TesterRoot.Add(tester.ToXmlTester());
            TesterRoot.Save(TesterPath);

            XElement testersId = new XElement("ID", tester.ID.ToString());

            ScheduleRoot.Add(new XElement("TesterSchedule", testersId, schedule.ToXmlSchedule()));
            ScheduleRoot.Save(SchedulePath);
        }