Correct. This is a exercism task. and yes, there are no variables given but that does not in my oponion means that I cannot add a instance or a class variable I myself was thinking about adding two instance variables named digits which hold the first 9 characters and a variable named controlDigit Can I do it then this way 1) Check if the string is empty , if so return false otherwise made a new object like this in IsValidIsbn isValidIsbn: aString    if aString isEmpty      ifTrue: [False]     ifFalse: [ self new                         digits := something.                         controlDigit := something.] I can also do what I think you are describing isValidIsbn: AString   if aString isEmpty       ifTrue: [ ^false]       ifFalse: [ISBN isValidIsbn10] and then as you describe so have a isValidISBN10 method in the class side where I do the check there so the class ISBN has the two instance variables digits and controlDigit Roelof