anchor.diff:  procedure ISBNVerifierTest.check_digit_of_X_should_not_be_used_for_0;
   begin
     Assert.IsFalse(TIsbn.isValid('3-598-21515-X'));
  +end;
  +
  +procedure ISBNVerifierTest.empty_ISBN;
  +begin
  +  Assert.IsFalse(TIsbn.isValid(''))
   end;
   
   procedure ISBNVerifierTest.invalid_character_in_isbn;
  
positive.code: interface
  uses
    DUnitX.TestFramework;
  
  type
    [TestFixture]
  
negative.code:     [Test]
      [Ignore]
      procedure check_digit_of_X_should_not_be_used_for_0;
  
      [Test]
      [Ignore]
      procedure empty_ISBN;
    end;
  
  implementation
  