public void Predecessor_ValidInput_ReturnsCorrectPostalCode(string postalCode, string postalCodePredecessor)
 {
     var code = new LIPostalCode(postalCode);
     var codePredecessor = new LIPostalCode(postalCodePredecessor);
     Assert.AreEqual(codePredecessor, code.Predecessor);
     Assert.AreEqual(codePredecessor.ToString(), code.Predecessor.ToString());
     Assert.AreEqual(codePredecessor.ToHumanReadableString(), code.Predecessor.ToHumanReadableString());
 }