 Conversely, if an examinee has mastered attribute A2, then it is expected that the examinee has mastered attribute A1 but not A3.The hierarchy contains two independent branches which share a common prerequisite – attribute A1.Aside from attribute A1, the first branch includes two additional attributes, A2 and A3, and the second branch includes a self-contained sub-hierarchy which includes attributes A4 through A9.