public DepartmentCloneable(string name, DepartmentCloneable parent)
 {
     Name   = name ?? throw new ArgumentNullException(nameof(name));
     Parent = parent;
 }
 public EmployeeCloneable(string name, DepartmentCloneable department)
 {
     Name       = name ?? throw new ArgumentNullException(nameof(name));
     Department = department ?? throw new ArgumentNullException(nameof(department));
 }