public IEnumerable<ProductItem> get_all_products_in(DepartmentItem department)
 {
     return Enumerable.Range(1, 10).Select(i => new ProductItem { name = i.ToString("Product " + " for department " + department.name) });
 }
 public IEnumerable<DepartmentItem> get_all_subdepartments_in(DepartmentItem department_item)
 {
     return Enumerable.Range(1, 10).Select(i => new DepartmentItem { id = i.ToString(), name = i.ToString("Sub department 0" + " for department " + department_item.name) });
 }
 public IEnumerable<DepartmentItem> get_all_subdepartments_in(DepartmentItem main_department) { throw new NotImplementedException(); }