// (an IllegalStateException is thrown if the state is invalid when calling asCustomerList).
CustomerList customers = customerRepository.findBySurname("Karlsson").asCustomerList();
// 2. Take the first customer and edit it in the GUI.
Customer customer = customers.get(0);
CustomerInGui customerInGui = customer.asCustomerInGui();
customerInGui.setAge("-1");
System.out.println("Edit in GUI, is valid: " + customerInGui.isValid());
customerInGui.setAge("20");
System.out.println("Edit in GUI, is valid: " + customerInGui.isValid());