anchor.diff:      [Test]
       [Ignore]
       procedure check_digit_of_X_should_not_be_used_for_0;
  +
  +    [Test]
  +    [Ignore]
  +    procedure empty_ISBN;
     end;
   
   implementation
  
positive.code: interface
  uses
    DUnitX.TestFramework;
  
  type
    [TestFixture]
  
negative.code: 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;
  