public void addnewcourse(course cours, Student st1, Student st2, Student st3)
        {//at least course must have 3 studnts , so manager add this three
         // and afterthat he can add more by addstudent method
            course co = cours;

            co.addstudent(st1);
            co.addstudent(st2);
            co.addstudent(st3);
        }
 public void addstudent_to_existcourse(Student st, course co)
 {
     co.addstudent(st);
 }