public ChangeHeadCommand(Group group,
                          Student newHead) : base(group)
 {
     this.previousHead = group.Head;
     this.newHead      = newHead;
 }
 public AddStudentCommand(Group group,
                          Student student) : base(group)
 {
     this.newStudent = student ??
                       throw new ArgumentNullException("student");
 }